he remained forever / he was immortal
Form I verb. Active participle: خَالِدٌ (remaining/abiding). خُلُودٌ (eternity/immortality). خَالِدِينَ فِيهَا is the most common description of Paradise/Hell.
The concept of khulūd (eternal abiding) in Paradise or Hell is central to Quranic eschatology and motivates moral accountability.