Active key is needed to convince Snake to open the Dungeon in Graveyard Keeper game. The quest is available at the very beginning of the game. In order to get the key, you need to go to the Astrologist and give him a skull.
In order to get a skull, you need the Hardspares skill. That's in the Anatomy and Alchemy branch. After getting it, you'll be able to get bones.
After getting the skull, go to the Astrologist. He's waiting in Sloth Day under the Lighthouse at Sealight. After giving him the skull, you will get the Keeper's key. Your next task is to get to Snake. You will receive Instructions for Key from him. In order to get Snake's trust and obtain the instructions, you must bring him 5 points of faith. You receive them during the mass.
In order to make an Active key out of the Keepers key, you need access to the church. This is unlocked by upgrading the graveyard - this is described on another page. You can celebrate masses in the church for which you earn faith points.
After getting 5 faith points, go to the Study table. Place the faith there, keepers key and the Instructions for the key, and you will create the Active key, which you'll be able to give to Snake.