What do you get for winning a Demand Fealty war in CK3?

The defender becomes the attacker's Vassal (patch 1.20.0.3).

What does the Demand Fealty casus belli cost?

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

Casus belli · Conquest

Demand Fealty

Demand Fealty is a casus belli in Crusader Kings III (conquest). If the attacker wins: The defender becomes the attacker's Vassal. If the defender wins: The defender keeps the contested titles, and the attacker must pay them war reparations. Declaring it costs piety and prestige. The war goal is worth 80% war score. Patch 1.20.0.3, from game/common/casus_belli_types/06_ce1_wars.txt.

Quick answers (3)

What do you get for winning a Demand Fealty war in CK3?: The defender becomes the attacker's Vassal (patch 1.20.0.3).
What happens on a white peace?: The defender keeps the contested titles.
What does the Demand Fealty casus belli cost?: piety and prestige; the amount depends on the titles, claims and distance, worked out when you declare.

How the war ends

Attacker wins

The defender becomes the attacker's Vassal

White peace

The defender keeps the contested titles.

Defender wins

The defender keeps the contested titles, and the attacker must pay them war reparations.

If the war becomes invalid

The … has ended: it was no longer valid to continue.

Who can declare it

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

Every conquest 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) (6 lines of game script)

scope:attacker = {
	has_character_modifier = fealty_demanded
}
NOT = {
	government_has_flag = government_is_landless_adventurer
}

As written in game/common/casus_belli_types/06_ce1_wars.txt.

Defender (allowed_against_character) (6 lines of game script)

scope:defender.highest_held_title_tier < scope:attacker.highest_held_title_tier
scope:defender = {
	any_neighboring_and_across_water_top_liege_realm_owner = {
		this = scope:attacker
	}
}

As written in game/common/casus_belli_types/06_ce1_wars.txt.

Cost to declare

Costs piety and 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 (19 lines of game script)

piety = {
	value = 0
	add = common_cb_impious_piety_cost
}
prestige = {
	value = 0
	add = {
		desc = CB_SIZE_OF_TARGET_REALM_COST
		scope:defender = {
			every_sub_realm_county = {
				add = {
					value = 40
				}
			}
		}
	}
	multiply = vassalization_cb_prestige_cost_mulitiplier
	multiply = common_cb_prestige_cost_multiplier
}

As written in game/common/casus_belli_types/06_ce1_wars.txt.

Source: game/common/casus_belli_types/06_ce1_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.