"If Dumbledore chooses to ignore the warnings the cards show -- again and again, no matter how I lay them out -- the Lightning-Struck Tower. Calamity. Disaster. Coming nearer all the time ..."
-- Sybill Trelawney (HBP25)
The Lightning-Struck Tower, also called simply The Tower, is the sixteenth card in the Major Acana of the Tarot. It signifies change, often abrupt or even violent. Trelawney reads the Tarot in order to divine the future. Just before the events which resulted in Dumbledore’s death atop the Astronomy Tower, Trelawney stated that she kept finding the Lightning-Struck Tower card in her readings, no matter how she laid them out. She interpreted the card as indicating calamity (HBP25).
Commentary
Notes
The image after Dumbledore's death of the Dark Mark floating ominously over the astronomy tower resembles the classic image of the tower with lightning and falling bodies. 
From the Web
The Tower card on Wikipedia