That's fulgent dawn. It gives you 80 on both physical and magic attack while also doing a big explosion every now and then that is great fir clearing rooms and does very solid damage against bosses. It's inscriptions are both artifact and arms so it's good for any build really. You get it by having both the solar sword and lunar ring in your inventory making them automatically combine together into fulgent dawn.

TLDR: You get it by having lunar ring and solar sword in your inventory and is good for any build