Really fun little campaign with a cool theme of taking control of enemy units, love it! My one wish is for the purchasable unit to be a regular Thotis and the level 0 one to be the one you get from vessels leveling up. First it would reduce the unit swarms that grow out of hand really quickly, second it would give more of a chance to see the level 2 units which I never quite managed to get, and third it may make the campaign a bit harder since you wouldn't get so much use out of your leadership bonus. A few less trees in the map design may also be nice, it's not always easy to catch the elves in unfavorable terrain and misses really hurt for drain focused units.
It's hard to evaluate the sound, aesthetic and play since so much of it comes from Wesnoth itself, but I enjoyed the new character art, the angel girl is hot and I want to corrupt her! A few unique sound effects and art between missions can really make the Thotis stand out as a new and unique entity in the world of Wesnoth.
It was cool to see enemy units each transforming into their own corrupted versions, and often giving a unique line of dialogue when they do. There's a lot of nice little touches in the project that show a real dedication to the concept. Like for example the descriptions of every unit and an extensive report on the culture and biology of Thotis that is incredibly easy to miss. I never paid much attention to such things in my own days of making custom Wesnoth content, but you've really given your own campaign a lot of thought!