AO's Dimension Beam

You weave your hands forming signs of forgotten languages giving you Truesight, and focus on a singular target that is within sight (the target can be one that appears visible to you only after you've gained the Truesight). The signs burn in a bright display of vivid colors. For each successful attack made against you while you are casting this spell, you must make a successful DC 15 saving throw, using your spellcasting modifier. The spell ends prematurely if you fail this saving throw (the spell slot is wasted).
During the first round of casting, five runes appear on the ground around you and in your space, one for each of the five elements (fire, earth, water, air, and lightning). If you are hit by an attack on the subsequent turns whilst still casting this spell, the attacker rolls a d6 before rolling damage for the attack. Apply the appropriate effect for the roll:

On the second turn of casting, a thunderstorm starts forming overhead and the nearby sky is turned completely black with colorful lightning flashing from the clouds - your turn ends as you continue casting.
On the final turn of casting a lightning strikes each of the five runes as you throw your hands forward, making a kaleidoscopic beam of pure magical force. Make a ranged spell attack against the target. On a hit the target dies instantly, obliterating not only their physical form but also their metaphysical being. A target killed by this spell cannot be resurrected by any means, not even by a god or a Wish spell. This attack can hit targets no matter what plane of existence they currently reside on during the final turn of this spell (e.g. a target that casts Plane shift during any of the previous turns will still be hit, even if they are no longer on the same plane as you).