Loggy's Inferno and Kokytos reverse engineering notes

Non-commanders die instantly (no resist)
The game checks every unit every turn. Units in Inferno take 1d20+ AP fire damage, those in Kokytos take 1d20+ AN cold damage.
In inferno: 50% to be attacked; attackers are 50% chance of 2d30+ imps, 50% chance of 1d8+ devils
In kokytos: 75% to be attacked:
1. 25% chance of 2d10+ unfrozen (1/4 total)
2. (if not unfrozen) 25% chance of 1d20+ winter wolves (3/16 total)
3. (if neither of those two) 1d8+ frost fiends (9/16 total)

Escape chance is 8% + plane returning bonuses
There is a 50% chance you come back in a province you control, otherwise it's entirely random
Returning has a 10% chance to give +1 fire or water (depending on the plane) to a max path of 4

Dominions 5 Wiki