Harrenhal burns
Harren the Black had spent forty years building the largest castle in Westeros, finishing it the very day Aegon landed, and he trusted his walls. Balerion went over them. The stone ran like candle wax with the Hoares still inside, and every lord in the riverlands promptly discovered they had always hated Harren. The ruin has been considered cursed by everyone who has held it since.
The fuller tale
Harren the Black had spent forty years and the lives of a generation of riverlanders building the largest castle ever raised in Westeros. Five towers, walls a horse could be ridden along, a godswood inside it big enough to get lost in. He finished it the very day Aegon landed, which is the kind of coincidence historians distrust and singers keep.
When Aegon came to the walls with an army Harren could have laughed at, Harren refused to yield on the reasonable ground that stone does not burn. Aegon did not attack the walls. He waited for dark, took Balerion up, and came down on the towers from above. What the walls did was hold the heat in.
Harren and all his sons died inside. House Hoare ended that night, and every riverlord who had spent three generations under ironborn rule promptly discovered that they had always been Aegon's men at heart ÔÇö which was true enough in the sense that mattered.
The cursed castle
Harrenhal has never prospered since, and its holders have a way of ending badly: Harroway, Towers, Strong, Lothston, Whent, and after them a string of caretakers who did not last long enough to redecorate. The rational explanation is that the place is far too large to garrison, far too expensive to keep, and was handed out repeatedly as a reward to men who had just made themselves powerful enemies. The other explanation is the one the smallfolk prefer.
It also gave Aegon his most useful demonstration: he had shown, on the biggest possible object, that the old rules of Westerosi warfare had been suspended.
Elsewhere in the wiki
 See the world as it stood in 1 BC What became of it  if an ending is written, it is here. Spoilers.
The chronicle records no ending here ÔÇö or none the maesters have set down.