Direct answer: the Ancestor Spirit is an optional boss in the southern Hallowhorn Grounds in Siofra River. To fight it you must first light eight flames on stone obelisks scattered across the region, then touch the horned remains south-east of the Siofra River Bank Site of Grace. Skipping the eight flames is the most common reason players cannot start this fight.
The name, before anything else
The boss is Ancestor Spirit, without the “-al”. A number of guides and wikis use “Ancestral Spirit” and that page does not exist. Note the trap runs the other way too: the talisman it rewards is “Ancestral Spirit’s Horn”, with the “-al”. Both forms are correct for different things.
Do not confuse it with the Regal Ancestor Spirit, a different and considerably harder boss in the northern Hallowhorn Grounds, up in Nokron. That one is only reachable after you beat Radahn.
The eight flames
Each flame sits on top of a stone obelisk. The first and easiest is a pillar with a brazier directly east of the Siofra River Bank Site of Grace — light that one and you have your reference point. The remaining seven are spread across the region, and several are in the north-west and near the Ancestral Woods.
Budget time for this. It is the most-skipped content in Siofra precisely because it is tedious to complete and the fight is optional. But it gates a boss that is one of the better fights in the game, and the reward is genuinely worth the walk.
The fight
A large open arena, which sounds worse than it is. The Ancestor Spirit jumps and hops while airborne, and fires a jet of blue energy from its mouth both on the ground and in the air. The mouth beam is the move to respect — it tracks and it hurts.
Use the Light of Miquella. The Ancestor Spirit is susceptible to it, and it shortens the fight substantially. If you have not picked it up, this is a good reason to.
What you get
Not a Greathorn. That belongs to the northern boss. This one gives the Ancestral Spirit’s Horn talisman, available from Finger Reader Enia in the Roundtable Hold for 3,000 runes on its own line in her inventory. The boss also drops runes directly.
Because the talisman is sold for a fixed rune cost rather than traded from a remembrance, you can buy it outright whether or not you beat the boss at all — but the fight is the point, and it is a good one.
Evidence status
Research-reviewed. The eight-flame unlock, the first brazier’s position relative to the Siofra River Bank grace, the remains location, the Light of Miquella susceptibility, and the Ancestral Spirit’s Horn reward and its 3,000 rune price were each cross-checked, with the reward confirmed against Enia’s full inventory table rather than a summary. Not gameplay-verified on camera.