In WC3: TFT, during the Undead campaign, Sylvanas (now possessing her physical body as a Dark Ranger) ambushes Arthas in Lordaeron and hits him with a paralyzing arrow. Arthas, due to his diminished strength owing to the Lich King weakening, is affected by this and left to Sylvanas and her cohort's tender mercies. Kel'Thuzad shows up and chases Sylvanas off, however; and Arthas makes his way to Northrend to heed the call of the Lich King and eventually become the Lich King himself.
This would be the incident where she "regains control," due to the first Lich King explicitly losing power to control the Scourge, and would also make sense of her stating she "betrayed her people," as in betrayed the Quel'dorei when Arthas raised her as a Banshee and she got yoked to the will of the Lich King.