What are the requirements for Elevate Self to Co-Emperor in CK3?

You: They … too preoccupied to act as … right now. (patch 1.20.1.2)

Will the AI accept Elevate Self to Co-Emperor?

Yes: it is accepted automatically.

Interaction · Diarch

Elevate Self to Co-Emperor

variesInfluenceAccepted automatically

Make a power grab, using the dominance of your Entrenched Regency to appoint yourself as Co-Emperor — dismissable only through death or maiming

What you need

You

To send it

… is not your Liege
Your liege:
- has_diarchy_active_parameter = diarch_can_appoint_self_co_emperor
They … too preoccupied to act as … right now

To see it in the menu

If played by the AI:
- is_diarch = yes
- Your liege:
  - Highest held Title is at least Empire Rank
  - Has the Acclamation Succession Law
  - One of these
    One of these
    Holds Byzantine Empire
    or
    Holds Roman Empire
    or
    Holds Eastern Roman Empire
    or
    Their culture: The Culture has the Administrative Emperors may enter into Duumvirates, voluntarily Sharing Power with another Character for powerful bonuses Tradition
- Loyalty less than 75
Your liege:
- has_diarchy_parameter = diarch_can_appoint_self_co_emperor
- Highest held Title is at least Empire Rank
- Has the Acclamation Succession Law
- One of these
  - One of these
    Holds Byzantine Empire
    or
    Holds Roman Empire
    or
    Holds Eastern Roman Empire
  - or
    Their culture: The Culture has the Administrative Emperors may enter into Duumvirates, voluntarily Sharing Power with another Character for powerful bonuses Tradition
is_diarch_of_target = liege

Elevate Self to Co-Emperor is a diarch character interaction in Crusader Kings III. You need: They … too preoccupied to act as … right now. Accepted automatically. It costs Influence (varies). Patch 1.20.1.2, from game/common/character_interactions/00_diarch_interactions.txt.

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Quick answers (3)

What are the requirements for Elevate Self to Co-Emperor in CK3?: You: They … too preoccupied to act as … right now. (patch 1.20.1.2)
Will the AI accept Elevate Self to Co-Emperor?: Yes: it is accepted automatically.
How much does Elevate Self to Co-Emperor cost?: Influence (varies) (patch 1.20.1.2).

Will they accept?

The other character has no choice: the game accepts it automatically.

What happens

Effects

The character you target:
- … Power Sharing transitions into a Duumvirate
- … Scales of Power swing is set to 40
- Loses 50 Legitimacy
- Opinion: Suspicion (-50)
If
- The character you target: Cannot rule by themselves
Then
- As … could rule by …, this action is substantially more expensive
Otherwise
- As … cannot rule by …, this action is substantially cheaper
Gain +150 Prestige

• the game's own tooltip text · ◦ our reading of the script line · code the script where neither exists

Who can use it and sees it (is_available, is_shown) (17 lines of game script)

is_available = trigger_if = {
	limit = {
		is_ai = yes
	}
	is_diarch = yes
	liege ?= {
		may_appoint_co_emperors_trigger = yes
	}
	diarch_loyalty < diarch_loyalty_visibly_loyal_threshold
}
liege ?= {
	has_diarchy_parameter = diarch_can_appoint_self_co_emperor
	may_appoint_co_emperors_trigger = yes
}
is_diarch_of_target = liege
exists = liege
is_shown = scope:actor != scope:recipient

As written in game/common/character_interactions/00_diarch_interactions.txt.

Can be sent when (is_valid…) (10 lines of game script)

custom_tooltip = {
	text = custom_description.tt.must_target_liege
	scope:recipient = scope:actor.liege
}
scope:actor.liege = {
	has_diarchy_active_parameter = diarch_can_appoint_self_co_emperor
}
diarch_cannot_be_disabled_trigger = {
	DIARCH = scope:actor
}

As written in game/common/character_interactions/00_diarch_interactions.txt.

Effects (on_send, on_accept, on_decline) (35 lines of game script)

on_accept = scope:recipient = {
	send_interface_toast = {
		type = msg_start_diarchy
		title = diarch_regent_appoints_self_co_emperor_interaction.tt.recipient_notification
		left_icon = scope:actor
		custom_tooltip = {
			text = diarch_regent_appoints_self_co_emperor_interaction.tt.become_co_emperorship
			set_diarchy_type = co_emperorship
		}
		custom_tooltip = {
			text = diarch_regent_appoints_self_co_emperor_interaction.tt.set_swing
			set_diarchy_swing = diarch_regent_appoints_self_co_emperor_interaction_sop_swing_set_value
		}
		add_legitimacy = minor_legitimacy_loss
	}
	add_opinion = {
		target = scope:actor
		modifier = suspicion_opinion
		opinion = -50
	}
}
if = {
	limit = {
		scope:recipient = {
			regency_for_personal_reasons_trigger = yes
		}
	}
	custom_tooltip = diarch_regent_appoints_self_co_emperor_interaction.tt.adults_cost_more
}
else = {
	custom_tooltip = diarch_regent_appoints_self_co_emperor_interaction.children_cost_less
}
scope:actor = {
	add_prestige = medium_prestige_gain
}

As written in game/common/character_interactions/00_diarch_interactions.txt.

Category: Diarch
Cost: Influence: varies
In the menu: main list
ID: diarch_regent_appoints_self_co_emperor_interaction

Source: game/common/character_interactions/00_diarch_interactions.txt and the English localisation, Crusader Kings III 1.20.1.2. “You” is the character who sends it (scope:actor, or a puppet acting for you); “the character you target” is the recipient.