Convergence

"The universe hasn't seen this marvel since before my watch began. Few can sense it, even fewer can see it. But while its effects can be dangerous, it is truly beautiful."

- Heimdall about the Convergence.

The Convergence, also known as The Alignment, is a phenomenal, cosmic event that is an aligning of the Nine Realms in Marvel lore and it occurs about every 5,000 years.

History
]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, the mad 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 his people, the Svartalfar (simply known as the Dark Elves), originated. And to make it happen, using a powerful weapon, a dark substance known as the Aether (which was later revealed to be one of the Infinity Stones). However, Malekith's attempts were thwarted by the Asgardians and their ruler Bor during the battle on the Dark Elves' homeworld of Svartalfheim. Following this, Malekith and the remaining Dark Elves entered a state of deep slumber for 5,000 years, until the time of the next Convergence, during which Malekith attempted his plan on Earth a second time, only to be stopped by Bor's grandson, the mighty Thor.