I tried using paragon late in the game and my souvereign didn't loose any more health after some casts. It seems like the spell only uses the base health. This is a real problem as you can use it nearly infinitely given enough mana.
Another problem is that experience multipliers seem to work on this skill too. This probably means that Paragon does not raise your level but gives you enough experience to reach the next level (or gives exactly the experience needed to fill that level, so if you have 490/500 xp it gives 500 and not 10 -> more useful - I didn't check which one it is).
Example: Souvereign is level 17 (and has 137 hp without any base hp left) and rest of army is level 50+ thanks to paragon
Seems to be this way at least since version 0.915:
https://forums.stardock.com/430142
https://forums.elementalgame.com/434753
https://forums.elementalgame.com/427670
Possible fix:
1a. Implement Paragon as health penalty so every form of health is used (should be displayed as: Paragon: -xy HP)
1b. Implement Paragon as it is now (see 2. and 3. for changes needed)
2a. If your Souvereign has not enough health he will die if you try to cast the spell (can be done as often as you want but will be expensive) and resurrect with 1 HP
2b. If your Souvereign has not enough health the spell can't be cast
3. The spell should give exactly one level (otherwise it's easy to get up to 4 levels with one cast if your army is big and has trainer skills!)
Possible methods to implement "one level":
1. Raise the level by one (...yeah thats as "easy" as it seems...) and keep the same experience for this next level (e.g. Level 10: 53/100 -> Level 11: 53/110)
2. Give the experience needed for the current level but don't multiply it (trainer skills etc.) <- same as 1. but different implementation
3. Give the experience needed to finish the current level but don't multiply it (trainer skills etc.)
for all three: trainer skills could minimize needed health to cast the spell, so if there is a multiplier of lets say +30% the spell should cost "ceil(5 / 1.3)"
