From Destinypedia, the Destiny wiki
Champions are special boss type enemies introduced in Shadowkeep. They appear in Heroic of higher difficulty content, such as Nightfalls or elected Lost Sectors, seasonal activities, raids starting from Garden of Salvation onwards in Destiny 2, an rarely in other activities. They are units who have been "touched" by the Darkness. Saint-14 stated that they were tricky enemies that he had fought multiple times in the Infinite Forest.
Champions come in three varieties; Barrier, Overload, and Unstoppable. A given enemy race has a Champion for two of the three types, and can be easily identified by red, tendril-like growths and dark coloration. These come in a crown shape for Barrier Champions, double horn shape for Unstoppable Champions, and two circles around the head and torso for Overload Champions.
Each type of Champion has a corresponding type of player weapon or ability that can be used to counter their special attribute. They will stun the Champion temporarily and will notify all nearby players with a sound and a text prompt. Anti-Champion capability can be most easily acquired through the current Seasonal Artifact with unlockable weapon mods and class ability mods equipped on armor. Some Exotic weapons also have Anti-Champion capabilities already attached.
List of Champions
List of appearances