The Rainbow Lich (5E)

From publisher blurb: A 5th Edition Mini-Dungeon adventure for 4-5 Characters Level 20 The elven wizard Ceatha chased rainbows in the misty skies and crafted a complex to aid in her transformation into a lich, yearning for everlasting life to view nature’s beauty. Her evil ritual channeled the power of the rainbow through a glorious waterfall, draining the powers of captured creatures and magical foci scattered through the dungeon. The ritual went horribly wrong, and now the elves report that rainbows come no more to the picturesque valley — and that corruption flows from the falls.