For a 5 head blast, you could have everyone within 500' roll 5d6.
If they get a 1 through 5 means they are hit with breath from that head (discard duplicates).
Then they take 10d8 of each energy type of each head that hit them, save for half.
That'll be brutal, but not 320 points of damage at the end of one turn brutal.
Simulating 10000 such rolls I get this:
0 heads 1 times
1 heads 195 times
2 heads 2278 times
3 heads 5106 times
4 heads 2268 times
5 heads 152 times
so basically an average of 3 heads breathing on each person, for 30d8, or 135 damage each. Some getting 4 heads for 180, some 3, and rarely 1 or 5.
0 is possible, but exceedingly rare (requires 5 6s).
Scale the "damage per head" up or down; I'd keep it uniform, just to avoid having to count out dice as often.
This could also cost less than 5 legendary actions, but would use up every head's ability to do a legendary action.
If they get a 1 through 5 means they are hit with breath from that head (discard duplicates).
Then they take 10d8 of each energy type of each head that hit them, save for half.
That'll be brutal, but not 320 points of damage at the end of one turn brutal.
Simulating 10000 such rolls I get this:
0 heads 1 times
1 heads 195 times
2 heads 2278 times
3 heads 5106 times
4 heads 2268 times
5 heads 152 times
so basically an average of 3 heads breathing on each person, for 30d8, or 135 damage each. Some getting 4 heads for 180, some 3, and rarely 1 or 5.
0 is possible, but exceedingly rare (requires 5 6s).
Scale the "damage per head" up or down; I'd keep it uniform, just to avoid having to count out dice as often.
This could also cost less than 5 legendary actions, but would use up every head's ability to do a legendary action.