To be Hidden, you must first successfully take the Hide ability.

This means the character must have some type of obscurement to ALL hostile creatures in the encounter (as creatures will communicate if they see a creature trying to hide).

This status effect (Hidden) is also a Hidden Roll to the Caster, this prevents metagaming, as the creature does not know if it is truly hidden or not.

The GM can see all status effects (visible, hidden, or secret).

Once the Caster is Hidden, the Caster CANNOT be the Target by hostile creatures, and is considered heavily obscured.

If successfully Hidden, the next Attack Roll the creature does, is considered at Advantage.

The GM may also grant Surprised if this is acted outside of combat/initiative.