If I attack with Empowered Akali against a Lonely Poro with Master Yi Wuju Bladesman as Legend, will the 2 damage trigger first or the Legend ability?

The Master Yi passive applies first — it's a continuous effect, not a triggered one.

Here's the sequence:

1. Attacker/Defender are established. Lonely Poro becomes the lone defender. Master Yi's "While a friendly unit defends alone, it gets +2 might" is a passive ability — it activates immediately when the condition is met, does not use the Chain, and doesn't wait for anything (per FAQ #9025 and #10960).
2. Lonely Poro's Might becomes 4 (2 base + 2 from Master Yi).
3. Then Akali's "When I move" trigger goes on the Chain. When it resolves, you may deal 2 damage to a unit at the battlefield.
4. 2 damage is not lethal to a 4-Might Lonely Poro — it survives. No Deathknell trigger.

So the 2 damage from Akali never gets a chance to "go first" — Master Yi's bonus is already applied by the time her trigger resolves. The Poro lives at 2 damage marked.