What does Tishana's Tidebinder actually target when used against a saga?
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The short answer
Tishana's Tidebinder targets the triggered ability on the stack (such as a chapter ability from a saga like Bahamut), not the permanent itself. Because that ability belongs to a creature, the permanent then loses all its abilities as a consequence of the ability being countered.
The card

Tishana's Tidebinder
Creature — Merfolk Wizard
Flash
When this creature enters, counter up to one target activated or triggered ability. If an ability of an artifact, creature, or planeswalker is countered this way, that permanent loses all abilities for as long as this creature remains on the battlefield. (Mana abilities can't be targeted.)
Illustrated by LeDania3/2
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