Red Heat
Eren is one of the few Alphas who go into rut without the presence of an Omega in heat nearby, and after suppressing it for nearly two years, the drugs are no longer working. As a last resort he heads to an Omega refuge, hoping to find a suitable, but temporary, mate. When he finds one lost in a fatal 'red heat', he'll do whatever it takes to ease his suffering.
When Levi's fist sounded out against the wood, the Alpha did not open it right away. Leaned into it, pressing his forehead against the smooth surface. Felt the pull of the Omega on the other side, just as he had that first day in the shelter. A whole new kind of gravity that tugged him not down to the earth but straight towards Levi. The laws of the universe shifting to bring them closer together, and Eren could not resist their inevitability.