The Nithilam are great monsters who can devour worlds. They gained access to every world when the Faerers opened a portal to a world they were in. Because the Faerers did not shut the portals behind them, the Nithilam were able to enter every world the Faerers had been to, up to and including Meliz, their homeworld.
Many of the Meliz Seraphim fled their world to the 'adjacent' Eretz, to the Stelian Isles. The Stelians had prepared for some form of apocalypse when the Faerers began their voyage, and were able to seal the portals leading to Eretz.
The Nithilam are monsters, vast as worlds. It is unknown if they are malicious in intent, or if they even have the sapience required to be malicious. However, they seem to only have one purpose- the consumption of all worlds.