Lord Rudra took birth as Ashwatthama, son of Dronacharya

अश्वत्थामशिवावतारवर्णनं नाम षट्त्रिंशोऽध्यायः

Ashwatthama incarnation of Lord Shiva Chapter 36

नन्दीश्वर उवाच |

Nandishwara said (to Sanatkumara)

अथावसरमासाद्य रुद्रः सर्वान्तकः प्रभुः |
स्वांशेन तनयो जज्ञे द्रोणस्य स महाबलः || 13 ||
अश्वत्थामेति विख्यातः स बभूव क्षितौ मुने | 14

At the proper moment, Lord Rudra, the destroyer of everything (in
Mahapralaya), with a small part of himself took birth as a son of
Dronacharya, of great strength. He became famous as Ashwatthama on the
earth.