tgp_join_house_bloc_effect

tgp_join_house_bloc_effect = {
	save_scope_as = joiner
	house_confederation ?= { save_scope_as = old_bloc }
	$INVITER$ ?= {
		save_scope_as = inviter
		house_confederation ?= { save_scope_as = bloc }
	}
	if = { # Tooltip for interaction
		limit = { NOT = { exists = scope:bloc } }
		scope:joiner.house_head = { custom_tooltip = join_house_bloc_tt_no_scope }
	}
	else_if = {
		limit = {
			exists = scope:joiner
			exists = scope:inviter
			exists = scope:bloc
			trigger_if = {
				limit = { exists = scope:old_bloc }
				scope:old_bloc != scope:bloc
			}
		}
		hidden_effect = {
			scope:old_bloc ?= { remove_confederation_member_house = scope:joiner }
		}
		# ADD MEMBER HOUSE
		custom_description = {
			text = join_house_bloc_effect
			subject = scope:joiner.house_head
			object = scope:inviter.house_head
			scope:bloc ?= {
				hidden_effect = { add_confederation_member_house = scope:joiner }
			}
		}
		# OPINION
		if = {
			limit = {
				$OPINION$ = flag:yes
				exists = scope:joiner
				scope:inviter != scope:joiner
			}
			scope:inviter ?= {
				house_head ?= {
					if = { # Hide for players
						limit = { is_ai = yes }
						if = {
							limit = {
								has_opinion_modifier = {
									modifier = left_house_bloc_opinion
									target = scope:joiner.house_head
								}
							}
							remove_opinion = {
								modifier = left_house_bloc_opinion
								target = scope:joiner.house_head
							}
						}
						add_opinion = {
							modifier = joined_house_bloc_opinion
							target = scope:joiner.house_head
							opinion = 15
						}
					}
				}
				change_house_relation_effect = {
					HOUSE = scope:joiner
					VALUE = house_relation_improve_medium_value
					REASON = joined_bloc
					CHAR = scope:joiner.house_head
					TARGET_CHAR = scope:inviter.house_head
					TITLE = scope:dummy_gender
				}
			}
		}
	}
}

As written in game/common/scripted_effects/10_dlc_tgp_house_bloc_scripted_effects.txt.