Multiplayer

The unifying principle

Death is cheap in this game; essence is the real stakes. So every multiplayer mode - co-op and competitive - must operate on the essence economy, not on kills. A PvP win that's just "I killed you" contradicts the entire single-player design. The interesting verbs are bearing, restoring, redirecting, and thinning the self.

Cooperative play - friendship as the willingness to bear each other's costs

• Shared tolls. When one player receives a boon, a co-op partner can offer one of their own faculties to cover one of the two required sacrifices. Sacrifice for a friend, literally - the cosmic balance applied to loyalty.
• Resurrection as self-sacrifice. When an ally falls, you can travel to the realm-between and bargain for their return - paying the toll from your own essence. Saving a friend costs you.
• Keepers of memory. Other players are the people who remember you. After a rebirth strips part of your self, a companion can restore a piece of your lost identity by telling you your own story - re-membering you, literally, through the spoken word. Friends as the wardens of who you are across deaths. (This is also the warmest expression of the word-as-power thread - naming and telling as restorative force.)
• Mutual geasa. Players swear binding vows to one another ("I vow never to let you fall"). Breaking a shared geas carries the same systemic consequences as any taboo - now interpersonal.
• Tribal complementarity. Because each tribe can offer what others can't (Corani aquatic relics, Fir Domnann ore, Tuatha de Danann an audience with Fae nobility), a mixed-tribe party can strike bargains no soloist could reach. Cooperation as expanded economic and spiritual range - emergent, and built straight from existing tribe design.

Competitive play - you don't kill a rival, you spend them

• The race to thin the soul. The win condition isn't a kill (death is cheap). It's eroding a rival's essence until they go to the sapling. A genuinely novel PvP victory: you outlast and out-bargain, not out-fight.
• Redirected tolls. A dark bargain that shifts your boon's cost onto a rival's essence - metaphysical theft. (Badb / warfare flavour.)
• Hastening fate. Raven's Sight (The Morrigan's Favor) lets you see death-timelines and hasten them - in PvP, accelerate a rival's slide toward the forest.
• Induced taboo. Manoeuvre a rival into a Broken Contract - sabotage through the spiritual economy, laying curses or the Mark of the Oathbreaker on them.
• Sovereignty. The Morrigan's Macha aspect (Regional Command) supports a light territorial/dominion contest - players vie for sovereignty over regions, shrines, and Fae alliances.
• Invasion. A player Blessed by the Morrigan can invade another's world as an agent of fate and war - Souls-like invasion, on-theme via the prestige class.

Possible hybrid

Because the world is animistic and forces, not foes, co-op and competition can coexist in a shared persistent world: you might bear one soul's toll and prey on another's, with the Fae economy mediating all of it. (Scope-heavy - note as ambition, not commitment.)