Spirius Review for Nahka

The concept of tweaking your abilities at the start of the game is interesting, but I'd remove the defensive stat boosts given by the passive; it might get to a point where you just kinda go "oh well" and pick the MR boosting one because the enemy team is full AP. Alternatively, you can consider giving something other than defenses when you pick Vasul, although that's just theoretical.
Not sure how I feel about their effects yet.

The other thing I have to point out is that it's pretty feels-bad when you can't heal your ally because the enemy just body-blocked your Q. For this case, skillshot heals should generally be unable to hit enemies; you can still run with this, though, by making it so that the AoE heals anyways. This'll make it so that it isn't COMPLETELY terrible, while keeping the idea of wanting to hit your allies with it instead of just throwing it out.
Not sure what kind of feedback I can give on the Emrin version.

Your W is interesting, but maybe increase the maximum channel time to 2 seconds so that you have a bigger amount of time to reactivate after you double the range.. It'll just feel better; no real reason otherwise.
One thing I'll note here is that Tenacity doesn't really sell the concept of mercy as well as, say, a shield or something; I'm envisioning this to be more of a longer ranged Mercy-esque dash ability, and Tenacity isn't what first comes to mind when you think about saving an ally. Just my personal opinion.
Last topic on Eternal Flight is the different cast paradigms, and some chatter about Emrin. It's weird that one of them needs an allied Champion and the other is location-targeted, considering they're supposed to be more-or-less the same ability, but with slight changes. For one thing, location-targeting gives us a ton more power (and, on top of that, a stun is usually more powerful than some Tenacity, not to mention it also gets to deal damage). Emrin's version of W is stronger in all situations, especially since you can dash to the location of an ally anyways and effectively save them by stunning people chasing them down/zoning them off. Emrin's version of this ability isn't an alternate choice based on how you want to play; it's more or less just better, since you can do what you can do prior and a then a bit more. If I were you, in terms of this ability, I'd find a way to encourage using Vasul to save allies and Emrin to dive into the enemy team to set up for their allies.
The core premise of this ability is good, and I think it's pretty cool; it just needs some tweaking.

I feel as though E is a pretty solid ability, although I'd scrap the damage reduction in favor of some movement speed; AoE damage reduction like that's a strong effect that you can't really see. The other thing you should take note of, though, is that a Spirius who's chosen Emrin can just tap E and sit on top of their allies to mess up any assassin who tries to dive onto their ADC. Not too important, and it can probably just stay (I might be wanting to specialize this too much), but meh.
The other thing I'd like to mention here is that it's potentially unengaging to just have a "hey you get shielded now!" zone around you for the duration. Idk.

The thematic idea of your ultimate is really cool, but I feel like it needs a large-scale overhaul/rework.
The first issue I take up with this ability is that the range is so huge it just doesn't really matter and you don't have to actively try and apply the stacks to people. It just happens.
The second issue I take up with it is that half the waves don't do anything; it's only the third that matters to people within 1000 units (a pretty big radius), since that'll give you your 5 stacks. That's effectively 3 seconds; for perspective, Taric's ultimate (although it's a 400 radius) gives complete invulnerability for 2.5 seconds after 2.5 seconds, and a % max HP shield is more effective on tanks, the people you don't have to worry about defending, than a squishy, who'd much rather a large flat health shield and would like to not have to build as much health.
Thirdly, it's nigh-impossible to escape such a long distance and avoid being silenced for 1.5 seconds. A 3 second delayed 1.5 second silence with a 1000 unit radius is really strong, and potentially way stronger than that shield. Something similar would by Zyra's ultimate, which has a radius of... Okay, the Wikia doesn't actually tell me for some reason, but Stranglethorns is, comparatively, a 2 second delay with a 1 second knockup.
Finally, that's a pretty big delay for your ability to actually come online. What is effectively a divine intervention would probably be faster and more urgent, and considering you can't attack or cast abilities for 5 whole seconds, that's gonna feel pretty bad for something like that. This is an ability with potentially very little counterplay and high impact, while also potentially not feeling very good to use.