A katakana noun meaning “hell,” most likely encountered in titles, names, games, fantasy settings, or direct renderings of foreign text. For ordinary religious or figurative Japanese, 地獄 is usually the natural word.
This game has a highest difficulty level called “Hell.”
ヘル is a word derived from English that means hell.
減る (へる) is an ordinary verb meaning 'to decrease,' unrelated in meaning to the katakana noun ヘル (hell) despite sharing the same pronunciation.
Only the katakana form ヘル. Its historical derivation is not specified; practically, it functions as a foreign-style katakana rendering of “hell.”