The answer to your question is really simple:
Quote from the rules:
During the regroup phase, players may perform regroup actions (special abilities on cards in play with "Regroup:" and events with that keyword) using the action procedure.
Detailled answer:
Special Abilities:
Besides events, other types of cards may have a phase action as a part of their game text called a special ability, which may be used only while the card is in play. (The boldfaced word defines when you may do so.)
Each special ability is optional; you don't have to use it if you don't want to. You may use each special ability as many times as you like (even repeatedly during the same phase), as long as you meet the requirements for it and pay its costs.
Phase Actions:
During each phase of a turn, one or more players are allowed to perform phase actions that use a word matching the name of that phase. These words are printed in boldface and followed by a colon.
Each phase action lasts for the duration of the phase named in the boldface word (unless otherwise specified).
The effects of a phase action with the keyword Skirmish: last only for the skirmish phase in which it is used.
Each phase action must be completely performed before another phase action can be performed. Phase actions cannot be combined.
An action labeled with the word "Response:" is not a phase action. Responses are explained later in this rulebook.
If Decipher wants you to discard it in a special phase they would use another wording like: At the beginning of the XYZ Phase discard a shadow condition for each Rohan token here. Discard this condition.