Most of the current SM weapons, mythical or not, aren't very unique in their abilities. Although the Super Nova has a special ability to annoy everyone, I can't think of any others. So here are some ideas:

Freeze Rockets
Mythical Item, 63 Weight
4-7 Range
15-20 Physical Damage
Special Ability: Disables enemy movement for 2 turns
-15 Energy, +15 Heat and -15 Rockets Per Use
2 Uses Per Battle

Goo Launcher
Mythical Item, 67 Weight
2-8 Range
Special Ability: Shoots goo that disables movement (lasts for 3 turns), covers 3 spaces
-12 Energy and +18 Heat Per Use
3 Uses Per Battle

Cluster Bomb
Mythical Item, 56 Weight
4-8 Range
35 Explosive Damage
+10 Heat Damage
Special Ability: Shoots 4 mini-bombs out on detonation that can deal extra damage up to 1 space away.
-8 Energy and +13 Heat Per Use

Tesla Cannon
Mythical Item, 52 Weight
4-6 Range
Special Ability:Disables one of the enemy's weapons for 1 turn. The weapon to disable will be randomly picked. Weapons that use more energy are more likely to be chosen.
-40 Energy and +15 Heat Per Use
1 Use Per Battle

Is this a good idea. Vote and comment.

nice idea.
how about this:
spikey (Want to be allies? Sometimes I like to pretend I am a princess riding a pony..) trolllalala
10-20 explosive damage
50 energy damage (?)
Special ability : make the enemy s damage inflicted became halfed for 2 turns
20 energy used
29 heat buildup
2 uses

bump