expression, noun
obscurity; dustbin of history; long-forgotten memory
Used for things that have been completely forgotten, often with a dramatic or literary tone. Common in phrases like 忘却の彼方に消える or 忘却の彼方へと追いやられる.
その事件は人々の記憶から消え、忘却の彼方へと追いやられた。
The incident faded from people's memories and was consigned to the dustbin of history.
かつての流行語も今では忘却の彼方だ。
Even the once-popular buzzwords are now long-forgotten.
A set phrase combining 忘却 (oblivion) and 彼方 (beyond, that side). The expression is not ancient but is a natural literary collocation.