Hitting a tree with sufficient cold damage makes it explode. The explosion type could be either physical, like a gas spore, or cold itself (which could possibly set off a chain reaction causing other trees to explode).

Dynamite in SLASH’EM should be changed so that it uses a digging explosion, and carves out tiles in its blast radius, rather than just a really powerful fiery explosion that doesn’t affect terrain. It should probably also create pits randomly in the blast radius.

Also, Archeologists should start with some dynamite (though perhaps they, or at least Lawful ones, ought to get alignment.penalties for using it, since blowing up your dig site is poor archeological practice).

Physical explosions (those from a gas spore) should blow away doors that are in the blast radius.

On deeper levels, land mines cause a fiery explosion in addition to their existing effects. Player-placed land mines get a bit trickier; a simple implementation will just consider the depth of the level the trap is on, but a more correct one would track via a field on the land mine object (in case it is picked up and carried around to another level) whether or not it will produce an explosion when triggered.

Flaming, freezing, and shocking spheres explode when killed like a gas spore does.

An artifact weapon that makes everything it kills explode (for not very much damage).

A SpliceHack spell to make splatters of blood have a use case. It causes blood splatters in a area scaling from 3x3 to 7x7 around the caster (depending on skill) explode, dealing physical damage to things caught in the explosions. Blood splatters on the floor are cleared, and new blood being caused by something dying might be suppressed if it becomes too easy to repeatedly cast this for lots of explosions.

As proposed, this would be a level 5 attack or matter spell.

Spellcasters who have summoned insects or nasties can sometimes also cast a spell that makes the summoned monsters explode at you.

If a digesting engulfer eats an e monster that explodes (either actively or upon death) or potentially any monster with an explosion attack, the monster is assumed to explode inside the engulfer, who takes double damage from that attack (or 8 times damage, if the explosion is considered to have hit them 8 times) and is stunned and immobilized for a few turns, unless they resisted the explosion’s damage type in which case they only suffer that for 1 turn.

This should also extend to other explosions inside engulfers (most of these would be player-created: e.g. reading a scroll of fire or breaking a wand while engulfed).

Exploding a bag of holding gives you a respectable amount of experience points (because you certainly learned something from that experience, and it would be hilarious to accidentally level up by doing this.)

Blowing up a bag of holding doesn’t destroy any of the items; it just destroys the bag.

Applying a cursed bag of tricks sometimes causes it to explode, causing magical explosion damage to you while also expending all of its create monster charges at once.

When a potion of oil gets hit by fire, it ignites and explodes.

Potions of acid have a splash radius effect when broken (could probably be implemented as an acid explosion).

When a bag of holding explodes, each item inside has a 1/13 chance of being destroyed and a 12/13 chance of being scattered (using the same code that scatters drawbridge chains). Note that scattered fragile items such as potions are liable to break by crashing into things.

Fire/flame nymph, n, which normally only spawn in Gehennom (or possibly near lava not in Gehennom) and are noticeably stronger than other nymphs. Have two steal attacks which burn any items stolen, and also a fire attack. Potentially a passive fire attack as well. They also explode into fire when killed.

Placing bag-of-holding-explosion-causing items into a bag of holding blows up the item that is trying to be placed inside, but does not harm the bag itself. If the item causing the explosion is a second bag of holding, its contents end up inside the outer bag.

Lightning spells:

• The most common suggestion is for a level 5 attack spell that creates a lightning bolt, implemented as a simple ray. Possibly, it causes an electrical explosion on the first target it hits if cast at Skilled and Expert.
• Alternatively, a spell of lightning bolt could be a melee range spell that deals heavy shock damage.
• Chain lightning is a spell that forks off and hits adjacent targets in a breadth-first search (see here for how it might work). Possibly, the lightning bolts can only chain off at 135 or 180 degree angles (maybe 90, too). Chain lightning can still be reflected, but becomes a straight ray.
• Lightning storm, which causes an electrical explosion centered on every monster in your line of sight (possibly with a decreasing chance of doing so as it gets further from you). Or if this is too overpowered, just strike these monsters with an individual lightning bolt that does not cause an explosion.

All spheres should explode in a 3x3 explosion like gas spores do. They should do this regardless of whether they died by attacking or some other method, so they should have an appropriate AT_BOOM attack.

Explosions destroy diggable walls.

Any sort of death by explosion should scatter the possessions around, rather than having a single bones pile stack.