。
He vanished without a trace.
Literal
He [topic-は] trace [also-も] not appearance [object-を] erased-[regret-てしまった-explanatory-んだ].
跡形もなく ('without a trace') is a fixed adverbial phrase: 跡形 ('trace, mark') + も + ない ('not even') in adverbial form なく. 姿を消す ('to disappear,' literally 'to erase one's figure') is the standard idiom for vanishing. ~てしまう adds a sense of completion and emotional regret/dramatic effect — 'gone, just like that.' んだ rounds it off with explanatory weight.