Even that (used to emphasize that a previously mentioned thing or situation is included or is not possible).
彼は約束を忘れていただけでなく、それすら謝らなかった。
Not only did he forget the promise, he didn't even apologize for that.
この企画は予算が少なく、重要な設備それすら買えない状態だ。
This project has so little budget that it can't even buy essential equipment.
手紙をもらったが、それすら読まなかった。
She received the letter, but she didn't even read it.