Taken from the Monster Legends Wiki:
"Forsaken monsters are a very special category of strong monsters. Unlike all legendary monsters, forsaken monster cells cannot be requested or donated in the Team Chat. Players will only be able to have one copy of each Forsaken monster. Elementium and Elemental Cells would not work to rank up these unique creatures. Only with their own cells will you be able to craft their eggs or bring their powers to the next level."
People HATED Lord Pumpseed for this. Warmasters have the same restrictions, but people don't dislike them. Why? Because unlike Lord Pumpseed and (I sincerely hope not) The Forsaken, Warmasters appear every month, where you can get closer to obtaining or empowering them with a resource that's technically free. Lord Pumpseed will probably appear again, but SP made it sound like he won't, and if The Forsaken end up exactly the same as him, that means big trouble. People love using their Elemental Cells/Elementium to rank their monsters, because it's a grueling journey, saving up a precious resource over a long time to eventually make one of their favorite monsters stronger - for no cash, but the effort it takes to get there makes the result extremely satisfying and blocks players from just spamming it, which is a win-win for both SP and the players - the more time players spend on the game, the more likely they are to actually buy things to maybe boost their progress. Direct means of asking for cash like Lord Pumpseed have always, and will always, be answered with hate.
However, it is not too late for SP. If they remake The Forsaken into something like Warmasters v2.0, then it will make things very interesting. For example, imagine that SP released all planned Forsaken monsters. Then, like Warmasters, they rerelease them in Mazes where players may only get their Cells (in small to middle amounts, just like Warmaster Dungeons). These events would coincide with actual Mazes, and players would be left with a choice - do they go for a new Maze monster, or do they choose to empower their Forsaken monsters? On one hand, you could be an active player and get cells for a new monster. On the other hand, you might be a higher leveled player or not a very active one, and might choose to go for The Forsaken - after all, they will eventually return, and you will eventually get to rank them up high enough.
My message: The Forsaken currently seem like a failure, but they have a lot of potential. It would be tragic to see it wasted.
How to Rune monsters, which monsters use which Runes, etc.