[Kingly_Krab]

The custom spell is the easy part really, you just have to write the encounter based on if the client has X buff or not they receive Y damage. This is just an example and definitely is not a final version to say the least, I'm just trying to show you how to do it.

sub EVENT_SPAWN {
    quest::setnexthpevent(80);
}

sub EVENT_HP {
    if ($hpevent ~~ [20, 40, 60, 80]) {
        Spell();
        quest::setnexthpevent(($hpevent - 20));
    }
}

sub Spell {
    my @hate = $npc->GetHateList();
    foreach my $hate_ent (@hate) {
        if ($hate_ent->GetEnt()->IsClient()) {
            if ($hate_ent->GetEnt()->FindBuff(X)) {
                $hate_ent->GetEnt()->Damage(100);
                $hate_ent->GetEnt()->Message(315, "You have been hit for 100 points of damage, mitigating 900 damage because you have X buff.");
            } else {
                $hate_ent->GetEnt()->Damage(1000);
                $hate_ent->GetEnt()->Message(315, "You have been hit for 1,000 points of damage because you did not have X buff.");
            }
        }
    }
}