http://www.champions-online-wiki.com..._Storm_Calling

This power summons multiple lightning bolts, each of which can hit up to 5 targets. Thus the power can hit more than 5 targets.

I went to the CO wiki and did a quick check, max targets for tier 3 and below aoe is 5, tier4 is 10. Some constant effects like pestilence and electric shield attack all in their aoe's. Some times it might seem like more because as a mob dies, another, if available, is immediately picked up.

An explaination of this was recently given for Startrek Online, and that rationale would apply there.

Startrek Online's "Scramble Sensors" currently hits all targets in the AoE. The devs mentioned that they might introduce a 5-target restriction on the power, since it seemed an oversight that it didn't have that. (An oversight that combined with everything else the power does, also makes it very "unfun" and powerful, though a target restriction might solve the problem, at least not without creating other issues.)

AoEs are computational expensive for the servers, especially if the powers themselves are already fairly complex (scramble sensors being a confuse, that is probably also the case, as it makes the AI change its behavior, and players start switching targets, and can shoot and heal both friends and allies), so target limitations do not just exist for balance reason (though also for that), but also to reduce the server strain.

If I remember, most AoE power hits 5 targets in the game, and T4 AoE powers hit up to 10 ennemies, beside for some exception like Ego Sleep, Etangle Mesh and Storm of Arrows, those hit 7 ennemies.

There are also other powers like Invocation of Storm Calling and Unstable Sigils wich can hit multiple time in AoE.
For instance the Unstable Sigils do 5 explosions on 5 targets, those can be different or the same resulting in more focused or spreaded damage, up to 25 strikes.
While Invocation of Storm Calling, this one can be considered as 5 consecutive fireballs, this skill will summon Lightning up to 5 people but each lightning has a small AoE dealing half damage from the primary target on 4 adjacent people near the primary one. Resulting in very high and fatal strikes when NPCs are close together, again this skill has a maximum cap of 25 strikes depending on how close those NPCs are located together.

There are also items granting powers that hit up to 10 people. The first one comming in my mind is the Center of Gravity wich is similar to Havoc Stomp with an increased range and target cap.

So if you are looking for the meanest AoE in game, Invocation of Storm Caling is your friend. It may be slow to cast, but it definitly kills everything around you when used correctly.

Thanks! Invocation of Storm Calling seems to be what I had in mind. Big sphere attack, and a maintain to take advantage of my Dex/Ego SS. Only thing I'm not thrilled about is that it's targeted. Am I right that if the target dies during the maintain, the power will turn off? Or does the power continue to seek out new targets as long as you maintain it (which is what I'm looking for)?

Most powers seek new targets. The only i remember that did not was Conflag and you needed burning rain,
but that was last year and has been fixed meanwhile.

Because 5 targets is the biggest single spawn in the open world, with maybe a few exceptions?

Yeah, but I was trying to find a power I could use in the Crisis type missions or boss type battles where enemies keep spawning and you get surrounded. I forget what the missions are called on the maps where you see a certain blue symbol, and it's an "area" type of mission that is repeatable? I went to one In Lemuria where I had to rescue some divers, which was not a problem, but in the next phase it generated a ton of enemies all at once. There also one in the Desert that swarms you with tons of enemies, and I've done an escalating raptor type of battle in Monster Island that works the same way. Hitting 5 at a time doesn't do any good if you're surrounded by 10+ enemies.

Now I know what you'll say - these aren't supposed to be solo'd. But I just got my first chracter to level 35, and I'd like to do these at least one time. The problem is that NO ONE DOES THEM, so you have no choice but to try it on your own.

Yeah, I forgot about Invocation. Man, I love that power...

Hey! Nobody mentioned Wall Of Ice??? Cool, I get to contribute. I haven't used it in a while, but it used to have an unlimited number of targets. According to the Wiki, it still does. Great power. Drop on Frozen Footsteps and run around!