Convergence

"You recall what I told you of the Convergence?" "Yes, the alignment of the worlds. It approaches, doesn't it?" "The universe hasn't seen this marvel since before my watch began. Few can sense even fewer can see it. A world that's infected can be dangerous. It is truly beautiful."

- Heimdall and Thor

The Convergence is a cosmic event that occurs about every 5,000 years. During the Convergence, the Nine Realms enter a specific form of alignment that causes the boundaries between each realm to become thin. This results in several spatial anomalies including shifts in gravity, and most notably the formation of invisible portals that allow objects to move between realms, occurring at random. As the Convergence reaches its apex several large portals will open in the sky above central locations in each realm, each portal acting as a window through which parts of each realm can be seen.

During a Convergence in the distant past, Malekith attempted to take advantage of the weakened boundaries between the realms to destroy the universe and return it to its natural state of darkness, from which the Dark Elves originated, using a powerful weapon known as the Aether. However, Malekith's attempts were thwarted by the Asgardians and their ruler Bor. Following this, Malekith and the remaining Dark Elves entered a state of deep sleep for 5,000 years, until the time of the next Convergence, during which Malekith attempted his plan a second time, only to be stopped by Bor's grandson Thor.