Conjunction used to present an alternative or consequence, often with a warning or strong suggestion. Common in both spoken and written Japanese.
Please hurry up, otherwise you'll be late.
Study harder. Or else you'll fail the exam.
Kanji spelling; rarely used in modern writing.
Kanji spelling with 無く; very rare.
Derived from さも (in that way) + なくば (classical conditional negative of ない). The form なくば is an older equivalent of modern なければ.