How do you get dark forms or Umbrabow in Aniimo?

There are two ways in, and both sit inside the same mode. Darkler variant eggs drop from the Egg Heist activity itself, so one route is simply playing and hoping the egg you extract is the one you want. The other is the currency. Egg Heist pays out a currency that accumulates across runs, and that currency can be traded for an Umbrabow egg. If drops have not gone your way, banking enough currency gets you there without depending on a lucky pull.

If you searched for "Darkler" and landed here, that is the same creature. The Umbrabow form was called Darkler during beta 2, and Darklers still appear in Egg Heist. Treat both words as pointing at the same dark form.

What Egg Heist asks of you

Operation: Egg Heist is a multiplayer mode introduced in the third beta, built around collecting exclusive Aniimo eggs. Its format is not settled. It has run as a competitive race between teams, as a PvP-only match in teams of three, as a real-time PvEvP mode in the Lost Isles where three-player squads fight monsters and rival players to gather and extract eggs and resources, as an objective fight between two sides, as a bracket of up to six three-player teams contesting a single objective, and as a cooperative mode with no PvP combat at all. Which of those you meet depends on the build you are playing.

Routes that will not get you one

The mode is also the only source for a broader set of rare eggs and Aniimo forms that exist nowhere else in the game, and dark forms are the clearest case of that. The ordinary egg pipeline does not help here. Hidden eggs revealed by burning overworld obstacles, eggs awarded for collecting Lumen Ambers, mysterious eggs earned from activities, eggs sent to you as gifts, and paid options like the $9.99 Advanced Pack with its Prismana egg all hatch other things. No amount of hatching outside Egg Heist produces a Darkler or an Umbrabow.

Not settled: The only route to a dark form runs through a mode whose format is still changing, so how much of it is PvP and how much is cooperative, and therefore what stands between you and an Umbrabow egg, is not fixed yet.