Chromaggus is an encounter inside the Blackrock Mountain set of Mercenary Bounties.
This bounty can be picked up from the Bounty Board and recommends a Party level of 30 to complete it.
You can unlock Chromaggus by first completing Vaelastrasz.
Completing this Bounty will unlock the Nefarian Bounty.
Changes Role at the End of Turn.
Potential Chromaggus 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 Chromaggus Bounty: