User blog comment:B214/Volume 23 discussion blog/@comment-2602:306:8075:390:ACFE:42D7:3070:DAE5-20170718181505/@comment-9595671-20170720122302

Yeah, it was ultimately the Chaos Break that did Sieg in, that and his demonic swords leaving him since they fused to him when he transformed, so leaving them likely left some serious damage to his body.

Had he not used it or fused with the demonic swords, he probably could have just healed with the Phoenix Tears and kept going for a bit longer.