A Critical Hit With Prismatic Beams...

[mneme]

Ok, so what about Prismatic Strike (Artificer e23)?

Personally, I think Hit * Critical = Critical Hit, so if the power explicitly says that the target can be hit multiple times from a single attack roll, then a critical can turn into multiple critical hits.

Mind, the wording on Prismatic Strike and Prismatic Beams are different, which confuses things a bit. PS explicitly says "the target can be hit 1, 2, or 3 times", wheras PB talks about how "target might be subject to any, all, or none of the effects depending on how many of its defenses were hit." So it's not entirely unreasonable to rule one way on PS + Critical and a different way on PB + Critical -- as the wording of PB is actually closer to the wording of the assassin's attacks that explicitly only ever "hit" once even with multiple hits.

 

[keterys]

It's only one roll, but it's compared three times so it's arguable either way as far as I can tell. Ie, "When you roll a 20 ... add the [Extra Damage]" suggests you do so once. The line "The target might be hit up to three times, depending on which defenses are hit." from Prismatic Strike suggests it's treated as three hits, so it could be three crits.

Again, RAI seems to be one crit, and WotC design has veered away from stacking damage several times on one thing lately, crit or otherwise.

 

[S'mon]

It's only one attack roll, so only 1 attack, so no way would I allow more than one application of bonus damage dice. If it involved 2 sorts of damage I'd let the player choose which sort the crit dice were.