Deepfathom Skulker

Devoid (This card has no color.)
Whenever a creature you control deals combat damage to a player, you may draw a card.
{3}{C}: Target creature can't be blocked this turn. ({C} represents colorless mana.)

Modern Horizons 3 Commander

Devoid (This card has no color.)
Whenever a creature you control deals combat damage to a player, you may draw a card.
{3}{C}: Target creature can't be blocked this turn.

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Oath of the Gatewatch

Activating Deepfathom Skulker’s last ability after it’s been legally blocked won’t change or undo that block. The declare attackers step is the last time you can activate the ability and have it be effective.