lethean

Of or relating to the river Lethe, one of the four rivers of Hades. Those who drank from it experienced complete forgetfulness.

They ferry over this Lethean sound.

The cup which was offered to you when you departed from Elysium was to be the Lethean draught, which would make you forget the joys you had experienced[.]

Of or relating to death or forgetfulness.

Men said that the garden had been roofed over against the sun with great sheets of lead and copper, leaving no cranny through which the tiniest star could peer down. Some swore that the privacy of its masters during their visits was ensured by a lethean slumber which Dwerulas, through his magic art, was wont to lay at such times upon the whole vicinity.