When Garen Commander (who gives other units +1 might at his location) moves to a battlefield, and the defender plays a reaction spell that does 1 damage to a unit with 1 might, does that unit die or survive with 1 HP?

Ruling: The unit survives with 1 HP. Garen's passive ability is always on and applies immediately when he arrives at the new location, giving the +1 might bonus before any reactions resolve.

Sequence:

• Garen moves to the battlefield (movement doesn't go on the chain)
• Garen's passive ability immediately applies at the new location, giving friendly units +1 might
• Attacking player passes on reactions
• Defending player plays reaction spell dealing 1 damage
• The unit has 2 might total (1 base + 1 from Garen), takes 1 damage, survives at 1 HP

Nuances:

• Passive abilities (worded as statements of fact) are always on and don't go on the chain, so they can't be reacted to
• Triggered abilities (worded "When X, do Y") and activated abilities (worded "Cost: Effect") do go on the chain and can be reacted to
• Movement itself doesn't go on the chain, so you can't react to the movement before passive abilities apply
• If the ability were worded "When I move here, other units have +1 might" (triggered ability), it would go on the chain and could be reacted to, resolving differently