Ilmirn
All Hydras/Cave Hydras/Acid Hydras lost in battle will be instantly returned to the Dungeon cities as an increased growth rate, provided the necessary structures are built.
NHF_Ilmirn_percent = 2; --The number of Hydras grown in the castle after one is lost in battle by a hero

Hydra multiplication is evident: Hydras appear immediately after being lost, with a ratio of 2/1 (appeared/killed). This is in addition to the growth rate. The effect is a large number of Hydras.

The point of the post is that a 1/1 ratio is quite good, almost like a Necromancer.