Final answer:
In the narrative of the Dream SMP, Eret was the one who betrayed L'Manburg by igniting a block of TNT, causing further explosions during the First War of L'Manburg.
Step-by-step explanation:
During the First War of L'Manburg in the Dream SMP, a fictional server storyline, it was Eret who betrayed L'Manburg and ignited a block of TNT which then triggered the explosion of more TNT that had been rigged. This act of betrayal led to a significant turning point in the storyline, with Eret switching sides to join the Dream Team. The Dream SMP server includes several scripted events, and this moment is one of the most memorable for fans of the server's narrative.