Alright, so I've finished Shadow of the Colossus HD and I have no idea what just happened at the end. I really confused. Help?

In order to fully understand the intracate plot of Shadow of the Colossus, you need to go and play the sequel.

Nothing much just one of the saddest ending to a video game Ever!!!

It's art, you just have to pretend you understand and it'll be okay.

Do this face for extra efficacy

It was all a metaphor for Last Guardian never coming out, ever.

It's a complicated yet simple game!

It turned out that Wander had the power of hope inside him all along.

In the end you realize that he is still dreaming.

I cannot stand this answer because it shows that whoever uses it didnt get the ending at all

I will xxplain in simple and short:

Wander enters the forbiddin land with the goal of asking  Dormin to ressurect Mono who was sacrificed for reasons not mentioned. A deal happens: you destroy 16 collossi each holding  a piece of my soul/body/essence whatever you want to call it and I will bring her back for a price..

wander defeats the 16 colossiand  he returns morphed by containing all the essence  of dormin. Wander did not know that the price was for mono's ressurection he would give his own body as a vessal for dormin( its hinted by dormin it was only temporary though). at the end the tribe of people that wander also belongs too reach the temple after figuring out wander took the sword and mono's body to the temple with intention of completing the ritual to free dormin. They come see him all shadow like and attemp to kill him but Dormin takes control and tries to return tthe favor but his injury slows him down( when wander was hit in the leg with a arrow) before the emon escapes he cast a banishing  spell that destroys/reseals dormin. Now  by accident or choice dormin decides to let wander keeps his body but degraded to an infant but with his essence  in his bloodline. he also keeps his promise to revive mono.

It was a win win situation, wander keeps his life mono is revived and Agro lives.. they live in the secret garden and thanks to ICO we know they eventually xxscape... why is it sad?

It just is sad! I almost cried when agro fell! Anyways thanks for the explanation.