If using ability rip, figure out how to explode first and then get nigh-infinite hp
If the DM says, "Since your PC now has the overdrive healing ability but can't explode, I guess your PC never will. Enjoy unlimited hp," then you can skip the rest of this answer. However, the 7th-level Sor/Wiz spell ability rip [trans] (Serpent Kingdoms 155), in part, says, that this "spell fails if used to transfer an ability to a creature that lacks the proper body parts, size, or other criterion for its use…."
With that in mind, I'd think it fair were a DM to rule that the criterion for benefiting from the thoon thrall's supernatural ability overdrive healing (Monster Manual V 111) is that a creature must be able explode. (This assumes that, absent other abilities to the contrary (see below), the saving throw DC to avoid exploding is the normal "10 + 1/2 the creature's HD + the creature's ability modifier (usually Charisma)" (Monster Manual 315). If the DM makes a different ruling, then you're on your own.)
It should come as no surprise that in Dungeons & Dragons 3.5 there are many creatures that explode. To dispense with the obvious, and because the thoon thrall's right there, the path of least resistance is first employing the ability rip spell to acquire the thoon thrall's supernatural ability immolate (ibid.) and then employing the ability rip spell again to acquire the overdrive healing ability. I assume, though, that the issue is that the explosion from the immolate ability kills the creature, and that's undesirable. So what's needed is a way to explode and not die.
I won't leave you hanging. From the same book as the thoon thrall there are at least two possibly safe options that can be gained with the ability rip spell:
- The ember guard's supernatural ability death throes says, "When killed, an ember guard explodes in a 60-foot-radius burst that deals 3d6 points of bludgeoning damage and 3d6 points of fire damage to everything in the area (Reflex DC 24 half). The save DC is Constitution-based" (MM5 52). Since the explosion triggers when the ember guard dies, it's not—and literally can't be—the explosion that kills the ember guard. Nonetheless, the specifics of the overdrive healing ability will (or should—we are, after all, in ask the DM territory) cause a creature with both abilities to explode yet suffer little or no harm for doing so. However, the creature will have undeniably still exploded, so To what degree does this explosion impair a still-alive creature with the death throes ability? is a question for the DM. On the other hand, the death throes ability is a HD 13 outsider's supernatural ability, so there's gotta be a way to get a hold of it if ability rip spells are already being tossed around.
- The bonespur's supernatural ability boneshard blast says, "Once per day, as a free action immediately following a charge, a bonespur can explode in a shower of bone shards. Creatures within 10 feet of the bonespur take 2d6 points of damage (Reflex DC 12 half). The shards of the bonespur transform into its normal towering shape at the beginning of its next turn" (MM5 156). Not only does this explosion explain how the creature reconstitutes itself after it explodes, the 6th-level Sor/Wiz spell create undead [necro] (Player's Handbook 215) can just create a bonespur from six humanoid corpses. This is by far a better option than the ember guard's death throes ability… if the DM rules that the specifics of exploding via the overdrive healing ability override the once-a-day limitation of the boneshard blast ability. If they don't, then the creature with both could find the overdrive healing ability nonfunctional after the day's first explosion.
I hadn't realized before now just how many monsters in the MM5 explode, but there are a lot, and I'm sure that better explosions from different monsters exist in other tomes. Since the Monster Manual V was already on the table, though, that seemed the best place to start. If allowed, Dumpster-dive into the campaign's permitted sources for more. Still, I would be terribly remiss were I to fail to mention one last option:
- The monstrous feat Final Strike (Savage Species 34) has as a prerequisite that the creature possess at least one of the air, cold, earth, fire, or water subtypes. (The 3.5 revision eliminated the acid and electricity subtypes. An elemental subtype like this can be gained from the Ritual of Association (SS 148) that costs the subject 36,000 gp and 1,440 XP.) When you, a creature that possesses the feat, are "killed (that is, when your hit points drop to –10 or lower), your body explodes in a final strike—a blast of elemental destruction—according to the table below," and that table for creatures with the earth subtype includes an earthquake effect! That's in addition to "[y]our final strike deal[ing] 1d6 points of damage per Hit Die, up to a maximum of 20d6. Each creature in the area may make a successful Reflex save (DC 10 + 1/2 your Hit Dice + your Con modifier) to halve the damage dealt." Again, like the death throes ability above, this doesn't kill the creature; death kills the creature. Sure, the feat's Special entry says, "A final strike renders your corpse unsuitable for raise dead or resurrection spells. Only true resurrection, miracle, or wish can restore life," but that's a problem for when you're a corpse not now when you're still alive! You do probably look awful, though.
Note: I have yet to play in a campaign wherein my PC kills creatures with the earth subtype, animates them as undead that possess the feat Final Strike via my PC's Graveborn Warrior feat from Dragon #312 (38), and sends his earthquake skeletons into battle, but I want to.
