While damsels are pretty mean little buggers, You may more of an issue going on here. Generally a damsel will nip at the fins of another fish until it is unable to swim well, they will then pick on it unit they (or stress) kil the fish. This will ussually take more then a day. The fact all of your fis are dying in one day leads me to suspect either poor water parameters or a poor acclimation proceedure. Can you post your current water parameters (ammonia, nitrite, nitrate, Ph, SG
, temp)? How long has the tank been running, how big is it and what fish have you lost? It is possible that the damsels are the root of the problem, but try not to get tunnel vision. You need to rule out some other options as well.