spine eren
Eren Yeager in his spinal state during the final battle of Shingeki no Kyojin.
Prior to Eren's transformation into the founding titan and starting the rumbling, Gabi Braun shot his head off and it touched Zeke Yeager at the same time, triggering the transformation. During it, his body was not regenerated and was instead simply attached to his titan with its spine.
This tag implicates eren_yeager (learn more).