Have you ever wondered why your druids increase the regeneration by 3 %, instead of the advertised 10 %?
In case you don't know everything about special abilities of units, you opened the right topic!
Courtesy of Tear.
1. "+/- X % of Y" abilities
Surely nobody can expect from one nightmare to decrease HP of all units of a huge army by 15 %. That would have to be a tough one.
In order these kinds of abilities to work at their maximum efficiency (for example, to reach 15 % in Aura of Despair), the ratio of your nightmares' upkeep to the overall upkeep of the inflicted army has to be at most 1:5.
Example: 100 nightmares (4000 upkeep) use their ability with full power when fighting against the army of overall upkeep of 20000 (or less).
If the enemy army is greater than that (e.g. 40000), the effect of ability decreases proportionally (7.5 %).
Don't forget that some (actually most) units with special (positive) abilities affect themselves as well.
Consequence: The ratio has to be at most 1:4 to the rest of (own or allied) units.
Example: 160 almuses (4000 upkeep) increase HP by 20 % if the upkeep of the whole army is 16000 or less. (Upkeep of whole army is 4000 + 16000 = 20000).
Hailstones (frost cannon)
To reach its full strength, the rule stands: The inflicted enemy army can be at most 5 times bigger in upkeep, to keep +15 damage per unit effect.
Frost cannon is ideal unit to crash armies with great amount of little HP units (skeletons, archers), because it inflicts additional 15 damage to each enemy unit. With nice army of frost cannons, feel free to choose necromancy experts in Arena and observe what happens.
Example, "comparable" strength:
200 frost cannons (4000 upkeep) against 4000 archers (4000 upkeep)
4000 archers do (16 x 4000) 64000 damage on cannons' 170000 HP (62% of cannons remain)
200 frost cannons do (200 x 400) 80000 damage, and thanks to Hailstones additional (15 x 4000 archers) 60000 = 140000 damage on archers' 140000 HP (0% of archers remain)
(Just for illustration; real battles have HP, damage bonuses and regeneration on both sides, of course.)
Finger of Death (death knight)
The exact opposite of cannons: dead knights are here to help us with many HP units (nightmare, behemoth..), because each knight has his own separate 10% chance to kill an enemy unit with the "Finger"
This is how it works exactly:
- Each knight separately (alive in the beginning of the round) can cast the Finger with 10 % chance. (Fingers are summed up, but not yet applied)
- Battle in progress, losses on both sides..
- Regeneration is applied, some units come to life again..
- And only now (in the very end of the round), Fingers (cast by knights alive in the beginning of the round) are applied. In case they are used on more unit types, they are distributed according unit amounts.(if 12 Fingers are used on 10 skeletons and 20 nightmares, skeletons get 4 and nightmares 8)
Example, "comparable" strength:
300 dead knights (6000 upkeep) against 20 hellbrutes (6000 upkeep)
(There is surely some exact damage & HP involved on both sides, but who cares: Knights have good chances to kill 34 hellbrutes in the round, while more than 2/3 of knights survives..)
If you have ever wondered, why do "Keeperlings" even exist, why there are not only Keepers in Forbidden Temples: Finger of death is the reason.
3. Death's Embrace (acolyte) 
This one is quite simple: If your acolyte dies in the round and regeneration is successfully applied on him, he rises as a different unit. (Necromancer)
So boost your common regeneration (capped at 50%), if you want to have more Necromancers. See regeneration for more on how regeneration is calculated and applied.
verifying the efficeny of an ability
You can verify how efficent a abilty was in the following way:
- go to the details of a battle
- select a round (even the fight was only 1 round you have to select round 1)
- you will see small blue bubbles in the lower right corner of the unit that has an ability.
- move the mouse curser over the bubbles and you see how efficent that ability of that unit was in that round.