If this face-up card you control is destroyed by your opponent's card (by battle or card effect) and sent to your Graveyard: Target 1 Special Summoned Level 5 or higher monster on the field; destroy that target.
Sacred Phoenix of Nephthys can be special summoned using Fire King Island, which can then be destroyed by Evilswarm Zahak's effect if it gets destroyed by your opponent's card. This will resurrect your Nephthys and destroy all spell and trap cards on the field, including your own Fire King Island.
This can provide some option if your opponent summons a high level monster with high attack and defense that you can't get over. Special summon your Evilswarm Zahak to your opponent's side of the field and attack into it, forcing your opponent to target their own monster.
Activates from your Graveyard
Destroys face-up Monster Cards / Destroys Special Summoned monsters
Zahak is the name of a person from a persian legend. That guy had snakes on his shoulders. Two snakes. Each had multi heads. This is easily a reference to that. He also killed his brother who was a king.