Can I react to irridescent avian TRIGGER with life from stone. First irridescent trigger enter the chain, I react with from stone, exhausting my unit, then irrisdescent Avian finishes granting the +3, draw and channel 1

Clarification that the Life from Stone cannot be used on Avian

Yes — you *can* respond to an ability once it's finalized on the chain, but only with a **Reaction** card/ability. Two corrections to the setup, though. Iridescent Avian's ability is an **activated** ability, not a trigger: "[T]: Buff a unit. If it's already buffed, give it +2 might this turn instead." It doesn't give +3 might, draw a card, or channel a rune — if you're getting that text from somewhere, you may be thinking of a different card. Activated abilities use the chain and also create a Closed State, so opponents do get to respond to it (Rule 377.3) — that part of your plan works. Also, exhausting one of your units in response doesn't stop the Avian ability. Its cost (exhausting the Avian) was already paid when it was activated, and choices/targets lock in at finalization, so the buff/+2 might still resolves even if the source or your board changes. Note: if you meant, can you pay the additional cost from Life from Stone with the activated ability of Iridescent Avian, then the answer is no, you can't. Like mentioned above, Avian's ability is a cost, so in order to put in on the chain, you need to pay that cost. Similarly with Life from Stone, you need to exhaust a friendly unit as an additional cost. You can't use that same unit in that scenario to pay both costs, unless you have a way to ready it before you would play Life from Stone.