If opponent has Mageseeker Warden on battlefield and I play red Darius, then kill Mageseeker with either a damage spell or a kill spell, will Darius untap?

Ruling: Darius will untap in both cases. Darius's ability triggers when a spell is played and goes on the chain regardless of whether Mageseeker is present. By the time Darius's trigger resolves, Mageseeker will already be dead from cleanup.

Sequence:

• Damage/kill spell resolves (marking damage or killing Mageseeker directly)
• Darius's trigger sees the played spell and goes onto the chain as pending
• Cleanup begins from the spell resolving
• During cleanup, Mageseeker dies (either from marked damage in step 2, or already dead from kill effect)
• Darius's trigger is finalized during cleanup (step 8)
• Darius's trigger eventually resolves with no Mageseeker present, allowing Darius to untap

Nuances:

• Mageseeker doesn't prevent these effects from entering the chain; it only causes them to resolve with no effect if Mageseeker is still present when they resolve
• The only difference between the damage spell and kill spell scenarios is when Mageseeker dies: damage spells mark damage that kills during cleanup step 2, while kill spells destroy the unit as part of the ability resolving
• Even if Mageseeker were present when Darius's ability resolves, Darius would still get +2 Might