Lethean

Pertaining to the river Lethe; hence, pertaining to or causing oblivion or forgetfulness of the past.

- Sir James Murray's New English Dictionary, 1908

Oblivious; [from] Lethe, one of the rivers of hell. From Greek letho, old form of lanthano, to forget.

- Daniel Lyons's American Dictionary of the English Language, 1897

Deadly, mortal, pestiferous.

- Thomas Blount's Glossographia, 1656