Re: Mercury, National Geographic had an issue this past year on poisons. Certain types of mercury are actually lethal in a relatively short time. In any event, it isn't exactly something I would want to use to fill my swimming pool.
As for weapons that depend on magic:
Blades that float and follow mental commands. IIRC, the "glaive" from Krull was sort of like this. One of the Monster Manuals had something called a Shardswarm, or a swarm of bladed constructs. Imagine something like that as a cloud that moves around the character, providing attacks and defensive bonuses.
How about a mirror or crystal ball that allows the user to cast Destruction or Disintegrate on the target? Kind of like the device in the old Star Trek episode "Mirror, Mirror."
What about rods that only cause damage to one type of creature? It would be fairly safe to arm a whole hoard of conscripts with things that only damage the undead, or goblinoids, or what have you.
What about a disease that polymorphs into a new disease on a daily basis?
How about adaptive armor that provides damage resistance based upon the last attack to hit it?
How about force nets or whips that cannot be broken or cut?
You could create a line of Swiss Army Knives, with different lengths and types of blades that magically fold out.
How about arrows or bolts that phase out when fired and phase back in inside the target? They would basically provide a ranged touch attack with no benefit from cover.
New spell: Teleporting Fireball.
Create a negative energy cannon that inflicts negative levels on targets. Victims who die rise as undead.
How about arrows that are carved to make a howling noise that results in a magical fear effect in a line to the target?
BTW, this is my 100th post. Whoo!