The Draconic Tree Sentinel is a Night Boss, meaning that it can appear at the end of a Day. It's a large armored knight on a mount that originally appeared in Elden Ring.
His signature attacks include wielding a hammer and shield in combination with stomps and lunges from atop his horse - he even has Fireball and Lightning attacks.
Although there are similarities in Nightreign, note that there are moveset differences compared to his original form.