Holy cow (or should I say moose?) this spirit is complicated. I'm pretty sure I've spent more time and effort analyzing Regal Ancestor Spirit than any other boss so far. I had to go back to the souls modding discord over and over again with detailed questions and deep mysteries to unravel. And after all that, this is what its combo chart looks like:
That's right, it's dead simple. I mean, this thing is a big hulking moose corpse—it's not exactly doing fancy sword dances. If it sees a problem it waves its antlers at it a bit or tries to step on it. Easy as.
So what gave me so much trouble?
This guy's AI proper is not so complicated. Where the complexity lies is in its event code, a separate system that drives everything from NPC subquests to elevator switches. It can also be used to implement certain aspects of boss logic, although nothing I've seen so far has been anywhere near as complex as this.
The thing about Regal Ancestor Spirit, and the reason I'm talking about it specifically rather than the earlier boss just named "Ancestor Spirit", is that it exists in an arena with ghostly animals wandering about. Unbeknownst to me when I played through it, these animals actually interact with a number of its moves in complex ways. Untangling that complexity so that I could accurately summarize the boss's real behavior on the wiki was my ultimate goal.
I'm not going to take you through the exact details of how it handles everything because it would take way too long, but I can run you through the high-level logic. When the battle starts, there are no animal spirits, but there is an event that triggers when the Ancestor Spirit uses one of its Spirit Mist attacks that spawns new animals (10 if there are no animals yet, 8 if there are fewer than 10, and 6 if there are more than 10). The most complex event function, which I beat my head against for hours, turns out to just be a gargantuan "if" statement that controls which type of animal spawns first.
(For the curious: in phase 1 it will spawn a deer first, in phase 2 it will spawn a boar and then a springhare, and in phase 3 it will spawn a goat. After there's at least one of its preferred animal, it will randomly choose from among three different lists of animals and spawn the next one on the list that hasn't already been spawned.)
The animal spirits on their own don't do anything. They have collision disabled, so they can't hit the player and the player can't hit them. But the Ancestor Spirit has other moves that interact with them in other ways. First, it has a move where it lies down and fades away. This triggers an event wherein the platoon AI for the animal spirits elects one of them to erupt into the Ancestor Spirit, killing the animal and giving the spirit a special effect ID indicating which animal type it was most recently reborn from.
It prefers to warp to the same types of animals it spawns: deer in phase 1, boars and springhares in phase 2, and goats in phase 3. It can in theory warp to other animal types, but in practice since it spawns animals so regularly and always keeps at least one of its preferred animal spawned, it'll pretty much always warp to a phase-appropriate animal.
Its next bit of special logic is actually almost entirely contained within its AI. When it falls below 50% health for the first time, the Ancestor Spirit will heal itself. This is clearly intended to look like it's siphoning power from the animals around it—it'll kill every animal in the arena (and damage you a bit if you stray too close) as power visibly flows in towards the spirit. But the amount it heals is static (50% of its max HP for the first heal which begins phase 2, 30% for the second heal which begins phase 3, and 15% thereafter).
Once it has healed and begun phase 2, it'll start warping to boars and springhares rather than just deer. And once it does that, it'll put into effect its real core gimmick: animal-specific attacks. I mentioned earlier that it records a special effect ID for its last animal type. The deer ID doesn't do anything, but if it has a boar ID it'll gain a new attack in which it puts its head down and charges the player, and if it has a springhare ID it'll gain one where it stands on its hind legs and hops like a bunny. If it manages to make it to phase 3 and get off a warp, it'll be able to do one of Elden Ring's characteristic goat rolls.
This whole complex web of systems to give this spirit an air of emerging from and being powered by the animal spirits is really cool to see... but I completely missed it when I played the game the first time, and I can't be the only one. I didn't even notice that the healing attack killed the animal spirits, since I totally tuned them out once I noticed that they couldn't hurt me. That one at least some players picked up on, but all the complex logic about taking on the attributes of the animal it emerges from feels almost wasted.
There's so much standing in the way of this mechanic being legible to the player:
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The Ancestor Spirit's arena is so huge and it warps so far away from the player that it's nearly impossible to see which animal gets reborn into a necro-moose.
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The deer doesn't confer any special move on the spirit, so the player's first encounter with the warp move is already divorced from any understanding of "gaining new moves".
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The boar charge is hard to connect to a boar specifically. Putting its antlers to the ground and charging seems well within range of a normal move the boss could do without any special modifier.
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The most distinctive move of all, the goat roll, is locked deep in phase 3 and could easily never be seen if the player just kills the Ancestor Spirit quickly enough.
The fight would be almost indistinguishable if they got rid of the animal spirits entirely and simply had the boss check its current phase to see which moves it could do. But at the same time, seeing the clockwork beneath the surface makes me wish it lived up to its full potential, that the logic here was actually material in a way that matters.
I suspect that was indeed the original plantoo. There's an unused animation in the game files in which its antlers grow larger, and it has a separate animation set for all its antler attacks with extra-large hitboxes. It's easy to imagine from there a deer-specific attack in which it does an jumbo-sized antler combo.
This alone would have done wonders for the legibility of the animal attacks. The fact that there's logic in place for resummoning more animals if enough are dead suggests that there may also have been a way for the player to interact with the animal spirits and exert some control on which moves the Ancestor Spirit took on. Perhaps at one point they weren't animal spirits at all, but flesh and blood that participated in the fight.
It's no secret that Elden Ring was pressed for time at the end of its development—and for such a massive game made by a relatively small studio, it's no surprise that things were left on the cutting room floor. But it's bittersweet to see the pale reflection of what could have been in what actually was. Regal Ancestor Spirit is a beautiful fight as-is, but it's not a terribly compelling one to play, and I think in another world it could have been.