You may have missed two updates before this.
His lips trembled with emotion as he dismounted his horse. He laid his sleeping son under the cool shade of a tree and covered him with his thick cloak. Legolas stirred restlessly and Thranduil soothed him with a touch of his hand and a gentle murmur. The child fell back into deep slumber, a thumb between his lips and moved no more.
Thranduil turned his attention away towards a circle of trees close to the river. A treasure lay there, as precious as his own son. Already the tears were filling his eyes, stinging them.
"Long have I stayed away," Thranduil whispered. He walked to the trees, his steps growing heavier. "I apologise, for I had not the courage to face thee again."
Under the shelter and safety of trees was a small mound the length of a small child. A large stone was erected at the head of the mound, marking the name, age and lineage of its occupant. Thranduil did not read it; he knew who it was. He collapsed at the foot of the mound and wept.
"Fear not, my son." His words were broken. He buried his fingers in the soft soil where his firstborn's feet were. "I am here, and you are now safe. No Orc shall ever harm you again." He sat there for as long as he dared, singing gentle lullabies by the grave, until Legolas awoke. Thranduil stirred when Legolas let loose a cry.
"Hush, little one," Thranduil murmured, gathering his son in his arms. "Let your brother sleep. It is not time for him to awaken... at least, not yet."
This was inspired by the famous lines in the Hobbit, where Bilbo Baggins was given a vest of mithril which would fit an Elven princeling. Thranduil's firstborn did not die by a dragon here but it was inspired by it.
Sorry (not sorry) for this really dark update. :P