What does the Unify The House casus belli cost?

prestige; the amount depends on the titles, claims and distance, worked out when you declare.

Casus belli · Claims

Unify The House

Unify The House is a casus belli in Crusader Kings III (claims). If the attacker wins: All of the defender's Vassals under the … House becomes the attacker's Vassals. If the defender wins: Any … House Vassals remain under the defender's rule. Declaring it costs prestige. The war goal is worth 80% war score. Patch 1.20.0.3, from game/common/casus_belli_types/05_fp3_wars.txt.

Quick answers (3)

What do you get for winning a Unify The House war in CK3?: All of the defender's Vassals under the … House becomes the attacker's Vassals (patch 1.20.0.3).
What happens on a white peace?: Any … House Vassals remain under the defender's rule.
What does the Unify The House casus belli cost?: prestige; the amount depends on the titles, claims and distance, worked out when you declare.

How the war ends

Attacker wins

All of the defender's Vassals under the … House becomes the attacker's Vassals.

White peace

Any … House Vassals remain under the defender's rule.

Defender wins

Any … House Vassals remain under the defender's rule.

If the war becomes invalid

The … has ended: the defender no longer has any vassals of House …

Who can declare it

The game shows no text of its own for these conditions; the script is below.

Every claims war (the group's allowed_for_character) (7 lines of game script)

herders_and_tributary_constraints = yes
pam_personal_pacifism_offensive_war_ban_trigger = yes
tgp_dynastic_cycle_offensive_wars_ban_trigger = yes
tgp_war_ban_all_ministers_trigger = yes
NOT = {
	government_has_flag = government_is_theocracy
}

As written in game/common/casus_belli_groups/00_casus_belli_groups.txt.

Attacker (allowed_for_character) (9 lines of game script)

is_house_head = yes
OR = {
	government_has_flag = government_is_clan
	has_perk = thicker_than_water_perk
}
is_independent_ruler = yes
NOT = {
	government_has_flag = government_is_landless_adventurer
}

As written in game/common/casus_belli_types/05_fp3_wars.txt.

Defender (allowed_against_character) (4 lines of game script)

any_vassal_or_below = {
	house ?= scope:attacker.house
	primary_title.tier >= tier_county
}

As written in game/common/casus_belli_types/05_fp3_wars.txt.

Cost to declare

Costs prestige. The amount is worked out when you declare (by the titles targeted, claims, distance and more), so the script is shown as written:

Cost (18 lines of game script)

prestige = {
	value = 0
	add = {
		scope:defender = {
			every_vassal_or_below = {
				limit = {
					house ?= scope:attacker.house
					primary_title.tier >= tier_county
				}
				add = {
					value = medium_prestige_value
				}
			}
		}
		desc = CB_BASE_COST
	}
	multiply = common_cb_prestige_cost_multiplier
}

As written in game/common/casus_belli_types/05_fp3_wars.txt.

Source: game/common/casus_belli_types/05_fp3_wars.txt and the English localisation, Crusader Kings III 1.20.0.3. “The attacker”, “the defender” stand where the game names the character; “…” marks other words it fills in during play.