mid-15c., "consequence of an event or action, a corollary; that which follows and forms a continuation," from Old French sequelle, sequele (14c.) and directly from Late Latin sequela "that which follows, result, consequence," from sequi "to follow, come after, follow after, attend, follow naturally" (from PIE root *sekw- (1) "to follow").
Specifically "a story that follows and continues another" by 1510s.
Also in Middle English "offspring, issue descendants;" also "train of followers, retinue." Beerbohm uses sequelula "a small sequel" (1912).