Fire Brand gives you extreme vulnerability to cold damage, making you take quadruple damage from it. If you have cold resistance, this only reduces it to triple damage.

Frost Brand works the same way with fire and fire resistance.

The game should not assume heroes all carry an unlimited invisible box of matches to light candles and lamps with. Doing that should require a fire source, whether that is Fire Brand, lava, a fire trap, the wand of fire, the fireball spell, or an already lit candle, lamp, or potion of oil. There could also be a new “lighter” tool which enables you to light things, but can get lost or run out of charges.

Consolidate Fire and Frost Brand (or allow them to be combined in-game) into a new artifact named something like “Frostflame Brand”. This weapon deals either fire damage or cold damage alternately on every hit.

To slightly encourage the use of underused types of dragon scale mail, have Fire Brand synergize with worn red dragon scale mail by allowing it to be invoked for a fireball, and have Frost Brand synergize with worn white dragon scale mail by allowing it to be invoked for a cone of cold.

A fake Fire Brand artifact, which looks and acts like Fire Brand but burns out in a couple thousand turns and thereafter works like a regular sword. It could be generated with gnome royalty in the Mines.

Dipping Frost Brand into water potions (or possibly any freezing-eligible potion, which is most of them) should freeze and shatter the potion. Likewise for Fire Brand, which should boil potions and ignite (but not explode) oil it’s dipped into.

Rather than being hardcoded as swords, Fire Brand and Frost Brand instead have a random base item type every game, chosen from the pool of melee weapons. Possibly the two Brands will always be the same type.

5% of killing hits with Fire Brand, on a monster that can leave a corpse, never do leave a corpse. This is accompanied by the message “The [monster] freezes solid and shatters!” or “The [monster] burns to ash!”

Note that this idea is NOT for the Brands to instakill monsters 5% of the time. It just blocks a corpse drop 5% of the time.

Fire and Frost Brand can be invoked for powerful fire and cold bolts.

When Fire and Frost Brand come into contact with water due to rust traps or dipping them in fountains, print a message about the water instantly evaporating or freezing. If either is dipped into a fountain, it dries it up.

Frost Brand is made of platinum and deals triple damage to monsters that are fire resistant but NOT cold resistant. Fire Brand is made of gold and deals triple damage to monsters that are cold resistant but NOT fire resistant.

Engraving with Fire Brand creates a burnt engraving as a wand of fire does (but still degrades the sharpness of the blade).