According to Isaiah 25:8, what will God swallow up?

Answer: Death

"He will swallow up death forever" (KJV: "in victory"), and the Lord GOD will wipe away the tears from every face.

“He will swallow up death forever. The Lord GOD will wipe away the tears from every face and remove the disgrace of His people from the whole earth. For the LORD has spoken.”— Isaiah 25:8 (BSB)