Hazrat Mahdi (as) will illuminate the whole world like the sun, and will be instrumental in all 'grief and oppression' coming to an end

HAZRAT MAHDI (AS) IS THE SUN WHO WILL DO AWAY WITH ALL GRIEF AND OPPRESSION. When he bestows goodness he is fertile rain. (Al-Barzanji, Al-Isha'ah li-ashrat al-sa'ah, p. 188)