Riftbound Buff

None of the 12 cards with Buff print a reminder explaining it, so there is no official wording to quote here. Rather than guess at the rule, the cards themselves are below.

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Which domains get it

Body4

Calm4

Chaos3

Fury2

Order1

Which sets print it

Unleashed · 12

Cards that mention Buff

These do not have Buff themselves, their rules text refers to it.

Adaptatron

When I conquer, you may kill a gear. If you do, buff me. (If I don't have a buff, I get a +1 might buff.)

Albus Ferros

When you play me, spend any number of buffs. For each buff spent, channel 1 rune exhausted.

Aphelios - Exalted

When you attach an Equipment to me, choose one that hasn't been chosen this turn —Ready 2 runes.Channel 1 rune exhausted.Buff a friendly unit.

Aphelios - Exalted

When you attach an Equipment to me, choose one that hasn't been chosen this turn —Ready 2 runes.Channel 1 rune exhausted.Buff a friendly unit.

Aphelios - Exalted

When you attach an Equipment to me, choose one that hasn't been chosen this turn —Ready 2 runes.Channel 1 rune exhausted.Buff a friendly unit.

Arena Bar

exhaust: Buff an exhausted friendly unit. (If it doesn't have a buff, it gets a +1 might buff.)

Buhru Captain

When you play me, you may draw 1 or buff me. (To buff a unit, give it a +1 might buff if it doesn't already have one.)

Call to Glory

[Reaction] (Play any time, even before spells and abilities resolve.) As you play this, you may spend a buff as an additional cost. If you do, ignore this spell's cost. Give a unit +3 might this turn.

Cithria of Cloudfield

When you play another unit, buff me. (If I don't have a buff, I get a +1 might buff.)

Fae Dragon

When you play me, buff up to four friendly units. (Give each a +1 might buff if it doesn't have one.) When you spend a buff, play a Gold gear token exhausted.

Fae Dragon (Spiritforged Nexus Night Promo)

When you play me, buff up to four friendly units. (Give each a +1 might buff if it doesn't have one.) When you spend a buff, play a Gold gear token exhausted.

Karma - Channeler

[Vision] (When you play me, look at the top card of your Main Deck. You may recycle it.) When you recycle one or more cards, buff a friendly unit. (If it doesn't have a buff, it gets a +1 might buff. Runes aren't cards.)