The party arrive at the lake, they do the whole stabby stab stab on Osred, Dungeon rises from the deep
Osred gets left behind and grabbed by the 'lady of the lake' and turned into the final boss EC
They enter down and discover tracks left by the previous acolytes
A corridor stretches off before them and makes several sharp turns, along the way are plenty of traps already set off
Following the corridors gets them to a central plinth with an item on it and behind it is the way out
Taking the item activates all of the previously disarmed traps
if they follow ahead they notice a blood trail
They enter a magical maze an 'infinite' amount of 10x10 rooms with four doors
The  blood trail on the floor leads between the rooms, following it brings them to the corpses of the previous acolytes, loot on them but once interacted with their twisted spirits rise and attack
The party can find a fluff way out or they shall be let out if particularly stuck, leading off to another corridor
If previously unencountered, the spirits attack here, else they are attacked by a construct of solid holy magic. An avatar of sorts. It cannot be reasoned with and it draws upon the 'vanquished' spirits from earlier, showing to be their master.
The avatar was shown to be defending a relic, one locked away with Merlyn due to its occultic power.
They approach another long corridor at the end is a grand, ornate door, leading to Merlyn proper, as they advance along it they don't seem to get any closer to the end and if they look behind them, they'll notice instead the way they came from has gotten further away. Trying to return causes the opposite to happen, trapping them in the middle.
They have to shatter a wall to the side to get through.
This leads them into another room which has a single, opened and empty chest in the middle. On the wall is a mirror, one that shows the chest to be full.
If they try to interact with the mirror a figure appears in the mirror world and starts to loot the chest. Before it turns to the party.
They all roll, the lowest gets attacked.
The figure leaves the room with a piece of loot, leaving them to try and figure it out on their own.
The mirror has to be shattered and the shards need to be placed within the chest to reveal the loot. If it's shattered with the figure in the room, he emerges and attacks the party
Finally the party emerges back round to the ornate door, opening it to enter Merlyn's tomb proper. Except that the area is a study and Merlyn seems to be quite happily writing away at a volume on his desk.
The room is lined with book shelves containing all kinds of 'forbidden' knowledge.
Merlyn is... pleasant, mainly. He is not entirely angry with them but he isn't exactly willing to help. If they try to challenge him, he will humiliate them (though without injury) and will question all of their true motives and reasonings.
Eventually he will give them a riddle of the Kaor blades hiding places.
After which he will shoo them out of the tomb, teleporting all of them to the surface.
Here they will face the empowered Osred who holds a holy relic sword
Osred will have some of his own reasoning but will be compelled to smite them and kill them by the spirit