If a Spend Setts buff to increase his Might to 8, can I trigger the Fiora legend?

Yes, spending Sett, Brawler's buff to increase his Might to 8 will trigger Fiora's Legend ability.

According to the official rulings for this interaction (RiftJudge FAQ #30, #528, #641, and #8727), this occurs because Sett drops below the "Mighty" threshold (5+ Might) before regaining it.

Sequence of Events:

1. Initial State: Sett is at 5 Might (4 base + 1 buff) and is currently "Mighty."
2. Activation: You activate Sett's ability: "Spend my buff: Give me +4 might this turn."
3. Cost Payment: As part of the cost to activate the ability, you spend (remove) the buff. Sett's Might drops from 5 to 4. He is no longer "Mighty."
4. Ability Resolution: The ability resolves, granting him +4 Might. His Might increases from 4 to 8.
5. Trigger: Because his Might changed from less than 5 (4) to 5 or greater (8), he "becomes Mighty" at that moment. This triggers Fiora's Legend ability.

Relevant Rules:

• Rule 709: "A Unit 'becomes Mighty' at the moment its Might changes from being less than 5 to being 5 or greater."
• Rule 708: "A Unit 'is Mighty' as long as its Might is 5 or greater."

Because Sett ceases to be Mighty when the buff is spent, he is eligible to "become Mighty" again when the +4 Might effect resolves, satisfying the condition for Fiora's ability.