King David mourned many days for his son (2Samuel 18)

2Samuel chapter 18 opens with David finally organizing his retaliation against Absalom. He apparently still has thousands of men on his side, and he divides them into three armies. The men tell him to not fight because his life is more important than theirs, and so he stays behind. 20,000 men are killed, probably mostly by David's soldiers. I think that verse eight is really saying that the slaughter was worse just because they were fighting in the forest, not that the forest literally killed people.

Now Absalom comes riding on a mule, and he gets his head caught in some tree branches. This is supposed to be ironic, because one of the things people admired Absalom for was his huge head of hair, and it proves to be his undoing. So somehow, he's left there by himself just hanging by his hair, and a man reports this to Joab, who gets angry with the man for not taking advantage of the situationa5 and killing Absalom. The man replies that David told everyone that no matter what happens, not to hurt Absalom. Joab clearly doesn't care, and stabs Absalom in the heart with three arrows; ten men with him also attack Absalom. Joab blows a trumpet and the fight ends. They throw Absalom's body in a pit and cover him with stones. I addressed whether Absalom had children in 2Samuel chapter 14.

A man wants to run to David and tell him the news, but Joab stops him, because he knows David won't be happy (and perhaps he wants to make sure the king doesn't know who struck the killing blow), but then he tells a different man to run and tell David. In the end, both men go, David finds out, and he appears to be heartbroken.