Feolthanos - 10-098L - Opus X

Summons or abilities of your opponent must choose Feolthanos if possible. When Feolthanos is chosen by your opponent's Summons of cost 5 or less or abilities of Characters of cost 5 or less, cancel its effect.

Official errata:

Summons and abilities of your opponent must choose Feolthanos if possible. When Feolthanos is chosen by your opponent's Summon of cost 5 or less or an ability of their Character of cost 5 or less, cancel its effect.