Luffy died in Wano, which allowed Nika to come to the fore and transform his corpse into a puppet that cackles with maniacal glee at the pain and suffering of those around it.
When Blackbeard touches Nika with his Yami powers it will be interesting to see what happens: it could result in a simple exorcism, drawing out the evil presence and breaking its hold on Luffy's body, allowing him at last to rest in peace, or alternatively and more hopefully, it could result in Luffy being given a second chance at life, escaping Nika's malign influence as though waking from a long nightmare.
What a dark story One Piece turned out to be. Grim stuff.