How does the Soul Siphon perk work?
What does this mean?
It means, you can take out your completely empty Sword of Flaming Flames, kill a bear with it, and now it's got 5% of the bear's soul for the next fight. (note: works for partially filled magic items too)
Does this require you to have an unused soul gem in inventory for it to work
Nope.
and how does it interact with a soul trap weapon?
- The soul trap weapon casts soul trap on the creature.
- The creature dies.
- 5% of the creature's soul is stored to the weapon.
- 100% of the creature's soul is stored to an empty gem.
Yes, that is 105% of the soul, which is awesome.
That description seems off, or at least misleading. The full perk description is:
Death blows to creatures, but not people, trap 5% of the victim's soul, recharging the weapon.
The Soul Siphon perk isn't meant to trap souls in the normal sense (like you would with Soul Trap), it's meant to be an automatic recharge for your enchanted weapon.
Skizzlefrits mentions in the comments how it works:
It would recharge the weapon for 5% of the amount that a soul gem filled with that soul would recharge. So this is a good perk to have at higher levels when you are fighting enemies that would fill grand soul gems.