Aegar, the Freezing Flame

Aegar's ability won't trigger when a battle is dealt excess damage. It's not his fault. They didn't exist yet.

Even 1 damage dealt to a creature from a source with deathtouch is considered lethal damage, so any amount greater than that will cause excess damage to be dealt, even if the total amount of damage isn't greater than the creature's toughness. Note that a source of damage having deathtouch has no effect on damage dealt to planeswalkers.

A creature has been dealt excess damage if one or more sources deal more damage to it than the minimum amount of damage required to be lethal damage. In most cases, this means damage greater than its toughness, but consider the damage already dealt to it that turn.