The spell Blink is a 3rd level spell (PHB 219) that essentially allows a player or creature to act like nightcrawler from X-Men.
Once the spell is cast (using an action), the player has a chance at the end of their turn to disappear to the Ethereal Plane. Then, at the start of their next turn they reappear within 10′ . This process keeps happening for 1 minute – and after the first casting, it does not cost any action, bonus action, or anything – It just happens.
So the sequence goes like this – attack, disappear, reappear, attack, disappear…
While on the Ethereal Plane, a character cannot be harmed or affected by anything on the material plane. Sweet!
Pro-Tip – Choose the dice that treat you well, cause it is just a straight roll – no modifiers – to determine if the spell goes off each turn
DM – Tip – Having the BBEG disappear like this after each turn leaves the players will little to do…and it is frustrating. If you give an opponent this ability, make sure there is something else for the players to do while that creature is blinked.