“Whether he’d manage to destroy it or not, he’d want to keep it hidden from Voldemort, wouldn’t he? Remember all those awful things we had to get rid of when we were here last time? That clock that shot bolts at everyone and those old robes that tried to strangle Ron; Regulus might have put them there to protect the locket’s hiding place ...“
-- Hermione Granger on Regulus Black and the Horcrux locket (DH10)
A very dangerous grandfather clock in Grimmauld Place “developed the unpleasant habit of shooting heavy bolts at passers-by.” Molly Weasley and the kids encountered the cursed clock while cleaning up the house. Remus Lupin helped to fix it (OP6). Hermione later wondered if Regulus Black had enchanted the clock to help protect Slytherin’s locket, a horcrux that Kreacher took from the sea cave when Regulus died (DH10).