同归于尽 is a four-character idiom meaning 'to perish together' or 'to end in mutual destruction'. It describes a situation where both sides in a conflict are destroyed, often used in contexts of war, rivalry, or extreme confrontation. The literal translation 'together return to the end' hints at the shared fate, but the actual meaning is that neither side survives. You can use it to describe a fight where both parties are wiped out, or metaphorically for a relationship or competition that destroys everyone involved.