Complex sentence: The priest upgrades to the "dark priest", that has the ability to drain health from enemies and also do a damaging area of effect spell that disables enemy priest from healing their allies, and causes enemies within range to temporarily lose control of their movement.
Split into simpler sentences: The priest upgrades to the "dark priest", who has the ability to drain health from enemies. The dark priest can also do a damaging area of effect spell that prevents enemy priests from healing their allies, and also causes enemies within range to temporarily lose control of their movement.