Mournwhelk — ruling 1

If you cast this card for its evoke cost, you may put the sacrifice trigger and the regular enters-the-battlefield trigger on the stack in either order. The one put on the stack last will resolve first.

Mournwhelk : règles, fonctionnement et interactions (MTG)

Mournwhelk 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

When this creature enters, target player discards two cards. Evoke (You may cast this spell for its evoke cost. If you do, it's sacrificed when it enters.)

How it works : Mournwhelk

Original English text — Wizards of the Coast / Scryfall

#1April 15, 2013· Wizards of the Coast
If you cast this card for its evoke cost, you may put the sacrifice trigger and the regular enters-the-battlefield trigger on the stack in either order. The one put on the stack last will resolve first.
No other card is named in this ruling.