IceBear
Explorer
You are allowed to add a few simple conditions to a readied action, so you could say "If they attack me or cast spells I will attack the leader". But if one whips out a wand, well, too bad.
I personally would allow, in that example, someone to say "If they take any hostile action towards me, I will attack the leader". I would then enforce a Wis check (DC=15) to have the character determine what is a hostile action (eg, The cultist drop their weapons, and one of them steps forward to attack you (actual he was going to step forward and kneel for mercy, but that's not how you saw it - failed the Wis check) so you kill the leader. No more hostage for you; the enraged cultists pick up their weapons and attack)
IceBear
I personally would allow, in that example, someone to say "If they take any hostile action towards me, I will attack the leader". I would then enforce a Wis check (DC=15) to have the character determine what is a hostile action (eg, The cultist drop their weapons, and one of them steps forward to attack you (actual he was going to step forward and kneel for mercy, but that's not how you saw it - failed the Wis check) so you kill the leader. No more hostage for you; the enraged cultists pick up their weapons and attack)
IceBear