It's suggested that the poison was the bacteria that causes botulism, which exists in the real world and is similarly deadly—a telltale sign of it is a bulge in the packaging of canned food. Syenite often lets her anger at Alabaster explode in the moment, and only in hindsight does she understand why he acts as he does. The sudden appearance of the obelisk—and most notably, that it is an amethyst obelisk, the same color as the crystal light from which Alabaster drew power—suggests that Syenite and Alabaster were just somehow
inside the obelisk using their orogeny. In response, it has moved physically closer to them.