Pinkie kills Death.
If Death were to be permanently destroyed, then all of existence would be altered. The Negative Energy Plane would slowly dissipate over the course of 320 days, with the Negative Quasi-Elemental Planes crumbling to nothing a full 160 days after that.
This would have massive repercussions throughout the cosmos. All undead beings would lose 1 hp per day once Death was destroyed, with this damage being permanent (unable to be restored by any means, save for a limited wish restoring 1 hp, or a wish restoring 1 hp per caster level). Those who managed to continue existing after the Negative Energy Plane was gone would need to make a Fortitude save (DC 368) each day or be immediately destroyed, in addition to now permanently losing 1 hp per hour. Living creatures, likewise, would gain a further death threshold of 1 hp below -10 per day after Death was destroyed (e.g. seven days after Death was destroyed, a living creature would only die when brought down to -17 hit points, etc). Once the Negative Energy Plane was completely gone, no living creature would ever die, no matter how far down their hit points dropped (similar conditions, such as having an ability score reduced to 0, would likewise just render a creature unconscious; not kill them).
It is likely that the forces of Existence would not allow this situation to stand, however. If Death's avatar still exists, it will likely find itself aided by various agents throughout the planes, hoping to grant it enough power to ascend to First One status and take the place of its previous incarnation. However, both Death's avatar and its helpers would likely have to contend with whatever was powerful enough to permanently destroy Death in the first place.