"We are not an empire, and I am not a queen. But I speak with the shout of the rivers and the whisper of the clinging mist."

Merfolk Sovereign

Other Merfolk creatures you control get +1/+1.
{T}: Target Merfolk creature can't be blocked this turn.

"Be like the sea: flow around that which is unmovable; for everything else, crash into it unrelentingly."

The Lost Caverns of Ixalan Commander

Jumpstart 2022

Explorers of Ixalan

Duel Decks: Merfolk vs. Goblins

Magic 2011

Other Merfolk creatures you control get +1/+1.
{T}: Target Merfolk creature is unblockable this turn.

To have any effect, Merfolk Sovereign's activated ability must be activated before the declare blockers step begins. Once a Merfolk has become blocked, activating Merfolk Sovereign's ability won't change that.