Sheol – שְׁאוֹל
Sheol is the lower and more difficult level in Gehennam that the souls go through after death, except for the Tzadikim. Souls can go up from this level to levels above it until they complete their cleansing process.
1 Samuel 2:6
“יְהוָה מֵמִית וּמְחַיֶּה מוֹרִיד שְׁאוֹל וַיָּעַל.”
“YHVH kills and makes alive; He brings down to the grave (Sheol) and brings up.”
A level below Sheol is called אבדון ‘Avadon’, which literally means ‘a lost place’. There is no going up from this level.
Teshuva at any level could save a wicked from Avadon.