Riddlekeeper — ruling 1

Riddlekeeper doesn’t impose a cost to attack. Creatures can attack you or a planeswalker you control even if their controller has no cards left in their library.

Riddlekeeper : règles, fonctionnement et interactions (MTG)

Riddlekeeper 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

Whenever a creature attacks you or a planeswalker you control, that creature's controller mills two cards.

How it works : Riddlekeeper

Original English text — Wizards of the Coast / Scryfall

#1September 22, 2011· Wizards of the Coast
Riddlekeeper doesn’t impose a cost to attack. Creatures can attack you or a planeswalker you control even if their controller has no cards left in their library.
No other card is named in this ruling.