Heretic

The Heretic level is a map to featured in the Mythic Map Pack along with 5 others. The word "Heretic" is a word used by the Covenant Loyalists to describe Covenant Separatists, hinting that this map may feature Covenant Separatist structures. Also it is known that this map will boast an Energy Sword. It can be assumed that this map will be good for Team Slayer.

Skull
The crest of the hidden skull on this level appears to be the Covenant, Mark of Shame. Judging by this seems the level will have a Covenant environment or the Flood controlled High Charity.