Are dreams considered part of the knowledge of the unseen (Ghaib)? Can a vision (Ru'ya) inform a person of their age or appointed time (ajal), or of the occurrence of something undesirable, despite visions generally being good?

The unseen is of two types: absolute, which none knows save Allah, and relative, which some may come to know through inspiration or visions. A vision may be clear or obscure, requiring interpretation, and it may indicate the approach of one's end, as happened to Umar ibn al-Khattab, may Allah be pleased with him.