It was a bit of a surprise but cope-able. It does seem to be a little random as to how long you can stand in one spot before it spawns. I found I usually could use 1 charge upgrading, move, use another, without attracting attention, but sometimes that got me another AoE. Also, and I'm not certain about this, I think it was nailing my beacon of despair and channeling damage at me, but there was such a big horde I couldn't be sure. I'm sure they'll monitor and tweak it appropriately.