Reverse Positive Energy Plane Effect

[BrokenChord]

Would it work by the rules, and if so is there a way, to pump an Undead with so many temporary hit points using negative energy that they would explode like living things do if they stay on the Positive Energy Plane too long?

[Chronos]

Unfortunately, the explosion calls for a Fort save and does not affect objects, which means that undead are immune to it. You can get around this using the Spark of Life spell, but that has a save, and if you're going that route, you might as well just hit them with a spell that will destroy them directly on a failed save.

A much better solution is to just houserule something, since it's universally recognized as stupid that undead can thrive on the Positive Energy Plane.

[TuggyNE]

Would it work by the rules, and if so is there a way, to pump an Undead with so many temporary hit points using negative energy that they would explode like living things do if they stay on the Positive Energy Plane too long?

No. The negative energy plane doesn't do negative energy damage and doesn't affect undead at all. The way I would houserule it is something like this:
Negative-Dominant
Planes with this trait are vast, empty reaches that suck the life out of travelers who cross them. They tend to be lonely, haunted planes, drained of color and filled with winds bearing the soft moans of those who died within them. As with positive-dominant planes, negative-dominant planes can be either minor or major. On minor negative-dominant planes, living creatures take 1d6 points of negative energy damage per round; undead instead gain fast healing 2 as an extraordinary ability. At 0 hit points or lower, they crumble into ash.

Major negative-dominant planes are even more severe. Each round, those within must make a DC 25 Fortitude save or be energy drained for one level; undead instead gain fast healing 5 as an extraordinary ability. A creature whose negative levels equal its current levels or Hit Dice is slain, becoming a wraith.

If you really want to you could put in something about temporary HP and a Will save for undead that have too many, but I'm not sure it's worth it.