The Ancestor Spirit is a boss located in the Siofra River Well that needs special conditions to unlock. To fight the Ancestor Spirit, you must first complete the Siofra River Well puzzle by lighting six flames in the area.
After completing the Siofra River Well puzzle, approach the dead beast in the Hallowhorn Grounds next to the Siofra River Bank Site of Grace. If the puzzle has been completed, a prompt will come up to touch the remains. Doing so will transport you to a cave area with a mist portal. Traverse the mist, and you will face the Ancestor Spirit.
The Ancestor Spirit looks like a giant moose-like creature that can unleash arcane fire. Its attacks are not particularly fast but can do a lot of damage. It will charge and headbutt a lot, which are great opportunities to roll under and attack. The Ancestor Spirit also has an attack where it will jump twice in the air and crash down, but will be open for attacks right after.
Bring some armor with high strike and magic damage negation. The Ancestor Spirit will occasionally jump and release arcane fire in the area or breathe arcane fire while on the ground. It is best to run far away while it is doing these moves.
Summoning a Spirit with ranged attacks is best against the Ancestor Spirit because the boss tends to move around a lot. If you do not have a Spirit with a ranged attack, ones with high health are helpful. If you defeat the Ancestor Spirit, you will be rewarded with a good amount of Runes and the Ancestor Follower Ashes, which are the spectral, minotaur-like enemies found in the Siofra River Well. The Ancestor Followers have a powerful ranged bow attack and can melee when up close.
For more Elden Ring guides, check out How to defeat the Spirit Caller Snail in Elden Ring on GameTips.PRO.