Mandatory effects (
Card such as "Caius the Shadow Monarch" and "Man-Eater Bug" have powerful effects, but because they are mandatory effects, if they are triggered when the opponent has no monsters, the player must target their own cards.
When a mandatory effect meets its activation conditions, it always activates, even if it has no valid target or it would have no effect. Mandatory effects cannot miss the timing. If a mandatory effect must activate during a particular phase or step, it is not possible to proceed to the next phase or step until that effect is activated.
When multiple effects trigger at the same time, they are all placed on a Chain, with the turn player's mandatory effects being activated first, followed by the non-turn player's mandatory effects. Optional effects can only be activated at higher Chain Links.