Only because it took me so long to figure this out:
Disciple Kosmas is rewarded when you reach Renown level 38 with the Kyrian Covenant*.
*there might be some other requirements, but I do not know them at this time.
Elite
Disciple Kosmas
The highest voted Wowhead comments for this page. They are community notes, not TheWoWDB comments.
Feels woefully undertuned as of writing this. I was really excited to recruit him due to him having Holy Nova, but it feels as if they forgot to give this guy a massive buff while they buffed the other Kyrian stuff to be on par, since he wasn't available until this week's reset. Just contrast him to Plaguey of the Necrolords for example - goddamn. It's a gargantuan difference in throughput.
As ZalQ noted, this companion is horribly undertuned. I have him at level 20 and his ability does not do much more than it did at level 1. I saw what it did when I first got him at renown 38 and was a little bit hyped, I figured he could end up being a really good champion. However this does not seem to be the case because as of now he does something like 25 holy damage and heals for 25 hp every 2 rounds. At level 20. This is terrible and has to be a tuning error, or am I missing something?
I guess I'll update this comment as he hits levels 30 and 40, etc. but I don't expect to see some dramatic leaps in his ability considering how little improvement it has undergone from 1 -> 20.
Details
This roster comes from the current game client's data files, so it is not a complete list of every NPC in the game - older world NPCs that are no longer in the modern client are absent.
Community notes from Wowhead · 9 comments
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Level and Map Tracking
The game data tunes Disciple Kosmas for levels 10 to 60, Shadowlands content.