Need a hand with a weapon enchantment (3.5, weapon effect, PEACH)

[Noctis Vigil]

OK, as some of you may have noticed, I'm in a prismatic kick right now (I have a reason). What I'm trying to do right now is make a prismatic weapon enchantment. However, I can't come up with anything that feels right to me. My current idea is a +5 weapon effect that treats a foe hit with the weapon as if they had been hit by the spell Prismatic Spray, or possibly on a successful critical (although I think the crit is only worth +4 if I go that way). Thoughts/suggestions?

[Vadskye]

Magic Item Compendium has this effect exactly. See page 40. For reasons unknown to me, they list it as a flat +30k gp rather than as a numerical bonus.

[Noctis Vigil]

Well so it is! Huh. Just as a ballpark guess, I'd put it as a +4 effect instead though. Does that sound good?

[Vadskye]

Applying a full +4 bonus for an effect that only happens on a crit feels too high to me. Crits aren't that common. I'd say +3.

[Noctis Vigil]

Isn't vorpal a +5 crit effect? I know lots of people who take that. Granted, prismatic effect isn't as good, but still has somewhere around a good chance to insta-kill, what with poison or turn to stone, plus the possibility of one of them on an 8; not sure the exact number, but I think it's in the ballpark of 30%; if we include the warp-to-another-plane bit, it jumps to around 45%. I seem to recall a buddy saying each effect has something like a 14.3% chance to occur.

Someone got a better head of numbers than I and want to crunch some numbers on that? I'd be interested in knowing the odds of insta-jib on the spell anyways.

[Vadskye]

Vorpal allows no saving throw. Prismatic Burst allows a DC 20 saving throw, and is only lethal roughly half the time. Assuming you're using a keen rapier, you'll threaten a critical on 30% of attacks, and confirm on roughly 15-20% of attacks. It is only lethal half the time, but it does damage the other times; let's assume that roughly corresponds to triggering on 10% of attacks. Since vorpal triggers on 5% of attacks, it seems better - until you take into account the saving throw to negate. That makes a huge difference.

So where does that leave us? That would seem to be a +4 bonus, roughly speaking. But don't forget that we're assuming a best-case scenario (keen rapier). The keen adds an extra +1 bonus, or the character takes a feat that requires BAB +8. Either way, it's a reasonable investment. I'd say that it's okay if prismatic burst is better on a keen rapier, while letting vorpal be better in general. Also, I don't think vorpal is actually worth a +5 bonus to begin with.

With all that in mind, I think I'm most comfortable with prismatic burst as a +3 weapon. I would never buy it at a +4, but I can imagine that some players would, so it would still be functional at that price.