Fiery Soul Trap

The Fiery Soul Trap enchant is a weapon enchant which, just as the name suggests, adds both a Soul Trap- and a Fire Damage-effect. It traps any creatures (or humanoids, if you use Black Soul Gems) soul if killed within 5 seconds after being hit with the weapon. In addition, it also deals 10 fire damage. (A Steel Battleaxe has been shown with a 12 damage version, but it is likely that the Augmented Flames perk in the Destruction tree is the cause of this increased damage, as this same perk seems to affect the Mace of Molag Bal similarly. Confirmation is needed.)  It has a purple glow effect. It can sometimes be found on weapons at random shopkeepers or as loot, albeit this is very rare. A possible, and most consistant, way of getting it is to go to Ironbind Barrow. A steel axe with this enchant sometimes sits on the back of the final boss' throne. If not, it might be floating in mid air somewhere in the room. In order to retrieve it, shoot it down with a spell.

Of particular note about this enchantment is the way it is treated at the enchanting alter. The variable effect is the duration of the Soul Trap, with the Fire Damage being fixed at 10 points. With this fixed damage in mind, one can set the duration of the Soul Trap to one second to have an extremely high number of charges with the bonus of 10 (or 12) points of Fire Damage. Couple this with another enchantment to do damage, and you'll have possible 35+ points of elemental damage as well as Soul Trap to refill the weapon.