The works of Ādi Śaṅkarācārya

Bṛhadāraṇyakopaniṣadbhāṣyam

Verse 363

तस्मादुच्यते मृत्युमृत्युमिति ।

In plain words

That is why he bears this name: Death's own Death.

Close to the Sanskrit

Therefore he is called 'the death of death.'