Run away!

You can't fight a black ooze with a rapier! Swords are their natural food and they enjoy clothes for dessert! No point hanging around to fight this thing... but you have a choice to make. The main tunnel you were following stretches away into the darkness and will probably lead you to safety... but there's also that smaller, half-hidden tunnel behind you. It might be a trap... or a way ahead that nobody would expect you to take!