To gain access to the dungeon you first need to complete the quest The Deepening Wake that you get from Eris Morn. The Pit of Heresey has a recommended power level of 940 and is full of puzzles and a difficult boss encounter. You will get rewarded with Pinnacle gear for completing this dungeon!
Pit Of Heresy
You must have unlocked the dungeon or selected the unlock option
You must be 930+ Power Level
You must have the Shadowkeep DLC