Handling
The Mark of Handling allows its heirs to transform wild beasts not only into domesticated animals, but into fantastic creatures. Each bearer of the Mark of Handling may bond with a single animal companion. This may be any friendly or charmed beast of challenge 1/4th or lower. This companion acts on the heir's initiative and may only take an action if the heir uses their action to direct it. The heir almost always then uses their mark to imbue the beast with magical abilities through the various manifestations of the Mark of Handling. Creatures might become swift and strong, sprout wings and learn to fly, or even develop a basilisk's deadly gaze. In addition to the overt powers granted by the manifestations of the mark, these beasts, known as mageborn animals, gain increased intelligence and hardiness. Those imbued with the lesser mark always have an intelligence of 3 if not already higher and at least five hit dice. Those imbued with the greater mark always have an intelligence of 10 and at least nine hit dice. Those imbued with the Siberys mark have been occasionally known to spontaneously learn the languages of those nearby.

Sadly, most beasts are only capable of temporary transformations under the Mark's influence, reverting to their normal form as soon as the bearer uses their power on another creature or travels more than a mile away. A rare few beasts can be permanently transformed, due to some quirk of genetics. Finding or breeding such a beast and them permanently imbuing it is an expensive and time consuming project, requiring resources  on par with the creation of a magic item. Treat those created by the least mark as common items for this purpose, those made by the lesser mark as uncommon, those made by the greater mark as rare and those benefiting from the Siberys mark as very rare. Permanent mageborn beasts may only benefit from one manifestation of each level, as their genetic affinity for one manifestation makes them incompatible with all others. Through this process, House Valdis has famously been credited for the creation of the first hippogriffs, griffins and hippocampi, while being blamed for existence of the owlbear.

Mageborn beasts, whether permanently imbued or not, occasionally pass their traits onto their offspring creating magebred animals who bear weaker but universally permanent versions of the abilities found in mageborn beasts. Magebred animals are of the monstrosity type and cannot receive further transformations through the Mark of Handling. An bearer of the Mark of Handling may earn 25 gold per day breeding magebred beasts, generally earning a lump sum twice a year when the eggs hatch or the foals are weened.

Least
1. You become proficient in land vehicles and gain the ability to imbue a beast as a mageborn mount as an action. The beast's size to become large if not already bigger, its strength to become 16 if not already higher and its fastest speed is increased to 60 if not already faster. Magebred inheritors of this ability are unusual docile, are always of least large size once fully grown, have a strength of 10 if it would not naturally be higher and have their land speed increased to 60 if not already faster. Both magebred and mageborn specimens gain an appropriate anatomy for use as a mount. You may have a maintain a single temporary mageborn beast with this ability at any given time.
2. You become proficient in animal handling and gain the ability to cross a beast with a spider to produce a mageborn arachnid as an action. These hybrid specimens gain the Spider Climb, Web Sense and Web abilities of a Giant Spider (see page 328 of the Monster Manual for details), but are thankfully no more venomous than the base creature would normally be. Magebred specimens have advantage on checks related to balancing and climbing. Both magebred and mageborn specimens gain the Web Walker ability of the Giant Spider and occasionally grow up to eight additional legs. You may have a maintain a single temporary mageborn beast with this ability at any given time.
3. You become proficient in water vehicles and gain the ability to imbue a beast as a mageborn amphibian as an action. The beast becomes able to breathe both water and air, and has a base land and swim speed equal to the base creature's land and swim speeds added together. Magebred beasts instead gain the ability to hold their breath for one hour, and have land and swim speeds equal to the better of of the two in the base creature. Both magebred and mageborn specimens often gain gills, lungs, legs, flippers and fins as appropriate. You may have a maintain a single temporary mageborn beast with this ability at any given time.
4. You always have the following spells prepared: Animal Friendship and Find Familiar. This has no effect and therefore may not be attuned if you lack 1st level or higher spell slots.
5. You may use your Dragonmark's ability to cast Speak with Animals three times per day instead of once. You now require no material components. You may not attune this option if you have not taken the Dragonmark Initiate feat.

Lesser
1. You have advantage on initiative so long as you are near your animal companion and gain the ability to imbue a beast as a mageborn tracker through a one hour ritual. The beast gains darkvision and hearing-dependent blindsight out to 60 feet and advantage on Perception checks that involve scent. Magebred trackers gain only one of these abilities, either darkvision, or blindsight or improved scent perception. Both mageborn and magebred trackers have their Wisdom increased to 18, if it is not higher already, and may develop exaggerated features relating to their affinity, such as enlarged ears, elongated snouts, or reflective eyes. You may only maintain a single temporary mageborn beast with this ability at any given time.
2. You gain advantage on any check used to maneuver a flying mount and the ability to imbue a beast as a mageborn flyer through a one hour ritual.. The beast gains a flight speed based on its size if it does not already have a faster flight speed. This starts out at 120 feet for a tiny beast, decreasing by 15 feet for each increase in size beyond tiny. A mageborn flying horse, for example, is large and has a flight speed of 75 feet. Magebred flyers are incapable of true flight. Instead, they are able to glide when they would normally fall, allowing them to slow their falling speed to 30 feet per round and move up to 60 feet horizontally while gliding. Both magebred and mageborn flyers are immune to falling damage and have their dexterity increased to 18 if it is not already higher. Specimens universally gain wings if they do not already have them, although they may be feathered, insect-like or leathery.  You may have a maintain a single temporary mageborn beast with this ability at any given time.
3. You gain the ability to imbue a beast as a mageborn monster through a one hour ritual. The beast gains natural weapons which allow it to make two attacks for 1d10 piercing, slashing, bludgeoning, poison or acid damage as a single action. Treat each attack as if it were made with a +2 magic weapon. The beast may not make any other attacks with this action. The shape the natural weapons take and the type of damage each attack inflicts is determined when this ability is used. At levels 11 and 17, the heir may imbue mageborn monsters with natural weapons which allow a third and fourth attack. Magebred monsters receive a single nonmagical natural attack instead. Both magebred and mageborn specimens have their Constitution score increased to 18 if it is not already higher and typically develop slavering fangs, giant claws, spikes, venom or acidic spit. You may have a maintain a single temporary mageborn beast with this ability at any given time.
4. You always have the following spells prepared: Beast Sense and Enhance Ability. This has no effect and therefore may not be attuned if you lack 2nd level or higher spell slots.
5. You may use your Dragonmark's ability to cast Beast Sense three times per day instead of once. You now require no material components. You may not attune this option if you have not taken the Dragonmark Initiate feat.

Greater
1. You become proficient in all armour made from chitin and gain the ability to imbue a beast as a mageborn chitinous beast through a one hour ritual. The beast develops a body covering layer of chitin, which counts as +3 platemail barding. Magebred specimens develop weaker chitin which counts as +1 half plate barding. The shed exoskeletons (or tanned hides) of can be used as barding for other creatures with a similar body structure, fashioned into barding for a different creature over the course of a week, or crafted by a leatherworker into armour appropriate for a humanoid over the course of a month. The shells of chitinous mageborn beasts can be used to make any form of +2 armour, while magebred shells can be used to make any nonmagical light or medium armour.  While most have insection exoskeletons, some beasts develop crab-like, or even snail-like shells. You may have a maintain a single temporary mageborn beast with this ability at any given time.
2. You may act normally while charmed and gain the ability to imbue a beast as a mageborn singer through a one hour ritual. The beast gains the ability to speak any three languages you know and a musical voice which is identical to that of the Luring Song ability of a harpy (see page 181 of the Monster Manual for details) save that it normally effects only beasts. Once per long rest, the beast may imbue its song with unusual poignancy, allowing it to instead effect all creatures for the next hour. Magebred beasts instead learn a single language and have their intelligence increased to 6 if it is not already higher. Both magebred and mageborn specimens gain beautiful singing voices and the Mimickry ability of a kenku (see page 164 of the Monster Manual for details). You may have a maintain a single temporary mageborn beast with this ability at any given time.
3. You ignore the False Appearance ability of creatures like darkmantles and animated shrubs and gain the ability to imbue a beast as a mageborn skulker through a one hour ritual. The beast gains an extreme form of camouflage which renders it completely invisible, even when moving or attacking. Magebred creatures instead gain a False Appearance (of the same type you are able to ignore), patterned after a common natural object in the region of the creature's birth, such as a rock, a specific kind of tree, a mushroom or a dead leaf. While stationary, the creature is indistinguishable from a normal example of the object. Both magebred and mageborn specimens gain advantage on Stealth checks.  You may have a maintain a single temporary mageborn beast with this ability at any given time.
4. You always have the following spells prepared: Phantom Steed and Fly. This has no effect and therefore may not be attuned if you lack 3rd level or higher spell slots.
5. You may use your Dragonmark's ability to cast Conjure Animals three times per day instead of once. You now require no material components. You may not attune this option if you have not taken the Dragonmark Initiate feat.

Siberys
You may elevate one animal as a sacred beast which blesses the region around it. All animals within 5 miles regain the maximum amount of health while resting, and all members of the same species are charmed by the sacred beast should they encounter it. The sacred beast itself is surrounded by an ethereal light, becomes immune to radiant damage and has its natural lifespan extended to ten years if it is not naturally longer. All sacred beasts have a copy of the Siberys Mark of Handling somewhere on their body. Upon death, the sacred beast returns to life as an adult member of its species. If it was slain by another creature, it gains the Regeneration, Rejuvenation and Vengeful Tracker abilities of a revenant, and must slay its killer within a year and a day or else cease to exist. If successfully defeats its killer, it loses these abilities and continues in its present form, still a sacred beast. If it had died of old age, the sacred beast is returned to life as an adolescent member of the same species which is just old enough to survive independently. Each of its ability scores are each increased by 2 to a maximum of 30 for physical ability scores and 20 for mental ability scores. The heir may not bless a new sacred beast while the another they have elevated still exists. A sacred beast's status can outlive the heir which uplifts it. Any children the sacred beast sires or bears are unusually healthy and exceptional members of their species, but are not sacred beasts themselves.