Counter-Summons (Su)

A Riftwarden can ready an action to use a summon monster spell or spell-like ability as a counterspell, even though the casting time for a summon monster spell is 1 full round. When used this way, the summon monster spell is treated as though quickened to a swift action via the Quicken Spell metamagic feat, though its spell level is not increased, and it can be used only to counterspell.

When used to counterspell, a summon monster spell can counter any conjuration (summoning) spell or spell-like ability of its level or lower, including an outsider’s summon ability.

At 5th level, this ability can be used as an immediate action, without the need to ready an action beforehand.