What it does
Addendum is a static ability on certain instants that gives a bonus effect if you cast the spell during your own main phase. You still cast it like a normal instant—it has flash-like timing flexibility—but you only get the extra value when you play it on your turn at a main phase, not during combat, an opponent's turn, or while the stack has other things resolving.
The common mistake: thinking Addendum cares about sorcery-speed timing in general. It specifically checks your main phase with an empty stack on your own turn. Casting it in response to something during your main phase won't qualify, and casting it on an opponent's turn or in combat just gives the base effect.
In Commander, Addendum cards reward proactive play but stay flexible as reactive answers. You sacrifice the bonus to hold them up for interaction, so weigh tempo versus the extra payoff each turn.
Iconic cards with Addendum
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