Group of assassins (their jobs, not their classes) contracted to kill the PCs. Greater invisibility while on the ethereal plane, and transdimensional spell. Just as they were waking up from sleep, finger of death the wizard, implosion the rogue, dominate the fighter, rhino's rush charging smite the cleric. I was bored and their reactions were worth it. Don't judge okay?
Some of these had to break etherealness to work but they still got surprise due to awareness rules.