Dominaria had survived calamities that would have ended younger worlds. The Brothers' War scarred the land and altered its climate. Later conflicts with Phyrexia devastated continents, while experiments around Tolaria treated time itself as a tool. Otaria's magical upheavals added still more strain. Each era left a mark, and the marks began to connect.
The result was not ordinary ruin. Temporal rifts opened above places tied to the worst disruptions. Through them, fragments of the past intruded on the present, possible futures bled backwards and familiar laws of magic became unreliable. Dominaria was slowly dying under the weight of its own history. The problem spread outward as the damage weakened the structure separating planes.
This is what makes the crisis different from another invasion. There was no enemy army that could simply be defeated. The danger was woven into locations, decisions and old victories. Closing a rift meant understanding why it existed and finding a sacrifice strong enough to answer the event that had created it.