"Bolters? Chainswords? All fine weapons, however I wield a far more dangerous power."

Epistolary Librarian

Veil of Time — Whenever this creature attacks, you may cast a spell with mana value X or less from your hand without paying its mana cost, where X is the number of attacking creatures.

Warhammer 40,000 Commander

Veil of Time — Whenever Epistolary Librarian attacks, you may cast a spell with mana value X or less from your hand without paying its mana cost, where X is the number of attacking creatures.

In Two-Headed giant and other formats where multiple players can attack at the same time, Epistolary Librarian's triggered ability counts all attacking creatures, even those controlled by other players.