you don't need to trigger the ent part of this.
just create an ability based on Raise dead, and change the following fields:
data - unit type
data - units summoned (the amount)
stats- targets allowed. - set this to [air, ground, dead, friend] (the settings from paladin's resurrection that only targets allies)
stats - buff - create a custom buff and set it here (otherwise the minion will have the "skeletal minion" timeout bar on it).
stats -duration - this is where you determine how long before the minion expires. if you want it to be permanent just set this to 0. (and then you don't need the buff)
and then just create a trigger that detects when a treant is spawned (and if you have other things that spawn treants like vanilla KOTG than make sure you have some conditions that differentiate.) and heal all friendly units in radius.
good luck!