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In Crawl, each of the Gods a player can worship gives them access to a set of monsters. These monsters are the types that player will see over the course of the game while trying to dispatch the human players. Each of the monsters can evolve, becoming stronger. For example, worshiping the god Murkan allows the player access to the rat, wurm, and shadow wraith enemies.
Each of these three monsters can be evolved by using wrath gathered based upon the number of levels the other players gain and what level you currently are. When enough wrath is gathered, and while in the menu screen in between stages, a player can spend available wrath to upgrade and evolve their monsters, making them stronger, and granting them special attacks.
The evolution paths are branching, allowing a player to see which possible evolutions are available for them to take ahead of time. A wurm might evolve into a giant fish, then into a witch, and finally, into a mindflayer, over the course of a single game.
Below is an example of a branching evolutionary path for the skeleton type monster, culminating in a dark lord.
It should be noted that the evolution tree of any monster links back into itself at the end. This is to say that a monster has a variety of different 1st and 2nd level monsters that it can become, but only 1 or 2 Peak Evolution monsters regardless of previous evolutions.
For example, the below picture shows some of the Skeleton Evolution Tree. While choosing the Skeleton Archer as the 1st tier would lead to a Flaming Skeleton Archer and the Necromancer, choosing the Skeleton Warrior leads to the Dark Knight and the Necromancer. From here however "Big Blue" is the only option for a Peak Evolution. (All of the monster names used here, aren't official, but are purely used for the sake of understanding.)