The player cannot encounter multiple rulers in the same mission. The first to appear is the Viper King, then the Berserker Queen and the last is the Archon King. The next ruler typically won't appear until the previous one is killed. That said, it is certainly possible to encounter the Rulers out of the usual order.
- A huge amount of health comparative to stage of the game the player typically meets them.
- Strong powers and abilities derived from the basic enemy they evolved from.
- They can perform one action after every action by one of the player's units.
- In XCOM 2: War of the Chosen, the Rulers' behaviors were altered to make them more manageable to deal with. In the expansion, each Ruler will only begin appearing on missions after they have first been encountered during an assault on an Avatar Project facility. Rulers will also only make a Ruler Reaction move after an XCOM solider within their sight moves or attacks. Neutral actions like reloading will not trigger a Ruler Reaction.
- They will try to escape via a psionic rift after a few turns or if they take too much damage.
- If they escape, the player may encounter them on another mission, with the same amount of hp they had when they escaped.
- Each Alien Ruler killed unlocks a new autopsy research that allows the development a new armor. The appearance and special abilities of each armor are directly linked to the associated Alien Ruler.
- Unlike most other loot items, Alien Ruler corpses are always recovered even when XCOM evacs from a mission.
- ...with the apparent exception of a base defense mission.
- The Burn and Acid Burn status effects are very effective against Alien Rulers; the duration is calculated by game phases (XCOM turn/Alien turn) but damage is dealt every time the unit performs an action. Significant damage can be inflicted in only a single turn by applying such a status effect at the beginning of the player's turn resulting in the Alien Ruler taking repeated burn/acid burn damage as it responds to each action done by the player's units.
- Similarly, Frozen, Stunned, and similar disabling status effects can prevent an Alien Ruler from activating based on player actions, but each attempted activation reduces their duration. This does not prevent damage status effects from ticking, though.
- It is possible for an Alien Ruler to be instantly killed by the activation of the Repeater weapon upgrade, so equipping squad members with repeaters can potentially bring a quick end to an unexpected ruler's arrival.
- A Psi Op's Stasis ability will prevent an Alien Ruler from taking Ruler Reactions for an entire turn. This allows for the player to position their troops as advantageously as possible without having to risk the Ruler moving, attacking or causing status ailments that might interfere with your strategy.
- Ending turns will not trigger the defrost tick on Alien rulers. This makes it possible to completely shut down a ruler by only attacking with Lightning Hands and ending turns to refresh the cooldown.