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The Empire created the green Adders after Serpents and Rattleserpents, when it transpired that both Rattleserpents and Serpents could essentially be rendered useless by being lured into dead ends. Luring an Adder into a completely sealed dead end will result in the delver not being able to dispose of the Adder, as the head is unreachable.
Unlike other serpents, Adders have enlarged heads which allow them to consume other monsters, with the exception of Rock Giants and all types of serpents.
Unfortunately, this dramatic increase in size made the head a vulnerable area.
Detailed Behaviour Analysis
As mentioned above, Adders move the same way as other serpents. They do not consider 1x1 tile monsters or rock piles to be obstacles and will eat them instead - the only exception there are NPCs with the Invulnerable imperative. They may also eat tarstuff providing it is a piece not vulnerable to stabbing. If they eat something, they grow by one square.
With each stab to the head, the Adder will shrink one tile. An Adder moving onto a sword does not take damage - damage is only done on the sword-wielders' turns. There are other methods of killing them, however: they will explode if a bomb or Fegundo explosion affects their head; they can be dropped from bridges. An Adder that is trapped will shrink if one of the possible squares it could've moved to contains an Adder piece (either its own or another Adder).
See also: Serpents and Rattleserpents