Corrupted Ancient Heroic is an encounter inside the Felwood set of Mercenary Bounties.
This bounty can be picked up from the Bounty Board and recommends a Party level of 29 to complete it.
You can unlock Corrupted Ancient Heroic by first completing Xavian Satyr.
After this survives damage, summon a @/@ Treant (improves each time).
Potential Corrupted Ancient Heroic Bounty Rewards
In addition to earning Mercenary Coins based on your Party members, Bounties are assigned to specific Mercenaries to allow you to target loot better.
The following Mercenaries can have their loot appear in the Corrupted Ancient Heroic Bounty: