Merfolk Sovereign — ruling 1

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.

Merfolk Sovereign : règles, fonctionnement et interactions (MTG)

Merfolk Sovereign is one of those cards players keep asking about. Here is, in plain words, what its official text says, the rulings published by Wizards of the Coast, and the combos players actually run.

Official card text

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

How it works : Merfolk Sovereign

Original English text — Wizards of the Coast / Scryfall

#1October 1, 2009· Wizards of the Coast
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.
No other card is named in this ruling.