My group and i have been playing D&D nearly 10 months now, each taking their turn at being DM (halfway through our 3rd campaign) and in the process have come to fall in love with some of our characters.
So as a treat i thought up the idea of bringing all our favorite characters together for one massive encounter, While still having a story line that fits in with all the different campaigns.
Story wise, i'm fine.
Encounter level not so much.
For the difference in some of the characters levels, i'm simply going to bump all of the PC's to LVL 21.
But because i have only done one campaign as DM before (and that was from a published adventure), and haven't even dealt with paragon tier yet, plus the fact that their are going to be 10 player characters, i feel i've bitten off more than i can chew.
Obviously i want the encounter to be challenging and exiting, so maybe simply use monsters higher in level than they are, but i don't wan't to go overboard, and decimate them (I have a back up plan if TPK, but don't want it to come to that.)
Any one have any basic rules of thumb for encounters like this?