Add a campaign script at the end of descr_strat.txt. In the campaign script, switch to the faction that should get the region using console_command control <faction>, console_command capture_settlement <settlement_name> to pass it to that faction, then switch back to the player faction. For example, let's say you want to give Rome to carthage:
Code:
declare_counter PlayerFaction
if I_LocalFaction carthage
set_counter PlayerFaction 1
end_if
if I_LocalFaction gauls
set_counter PlayerFaction 2
end_if
; etc, for all other factions
if I_CompareCounter PlayerFaction != 1
console_command control carthage
console_command capture_settlement Rome
; pass control back to the player faction
if I_CompareCounter PlayerFaction = 2
console_command control gauls
end_if
; do the same check for all the other factions (except the one you gave the settlement to)
end_if