- The "Mana Drain" ability will be weakened to 1 mana for every 20 Imps (instead of 10), rounded up.
This is a rare case where an ability is weakened so that Imps remain a decent melee unit (with ultimately high damage) and don't pay too high a price for an ability that is only useful against a hero and is useless for clearing neutral creatures.
In duels, this means that a full squad of Imps will drain 3 mana (previously 5), which is enough to allow Balrogs to cast the powerful Disintegrate spell (dealing half-stack damage) on their first turn.
However, if at least 10 Imps (20% of the squad) are killed before the Imps' first turn, then the mana drain will be only 2 mana, and the Balrogs will not be able to cast Disintegrate on their first turn.
If the Imps are not touched at all, then this mana drain rate is enough for the Balrogs to cast Meteor on almost every turn until the enemy hero's mana is depleted.
Thus, 5 mana per turn was excessive in terms of providing mana for the Balrogs, and from the enemy hero's point of view, it burned not only a lot of mana but also too quickly (while the Imps themselves remained a very decent attack unit, and it was better not to get within their attack range).
While 3 mana per turn is enough to create a threat of completely depleting the opponent's mana if they ignore the Imps, but not as quickly as before.