Attack on Titan vol 19

Eren and his friends confront Reiner and defeat him using a new weapon designed specifically to penetrate his armored Titan form, the "Thunder Spear". With his friend in danger, Bertolt joins the battle in an attempt to rescue him. After Armin attempts to dissuade him to no avail, Bertolt discovers that Reiner transferred his mind to his Titan body as a last resort to keep it functioning temporarily and transforms into the Colossal Titan determined to avenge his comrade.