Ritual of Soot
Sorcery
Destroy all creatures with converted mana cost 3 or less.
Only the patrol's armor was found, so tainted with the acrid smell of sudden death that it could never be worn again.
Illustrated by
Guilds of Ravnica Promos#84pRareEnglishLegality
Rulings
Oct 5, 2018
Tokens that aren’t a copy of something else don’t have a mana cost. Anything without a mana cost normally has a converted mana cost of 0.
Oct 5, 2018
If a permanent has 
in its mana cost, X is considered to be 0.