Pay-off what you set-up. Look at your first sentence and paragraph. What do you expect based on reading those lines. That is what the end of your story needs to deliver. The reader comes to you for an emotional experience, quick and sharp, and you deliver that in those final words.
Set a timer to three minutes. Then, for that time, without pause or editing, write eight last sentences to your story. Don’t pause, don’t think. Just write. One, two and three are easy. Seven and eight are difficult, but totally worth the effort.
It is very likely one of those sentences is the ending that you need.
Today’s writing (first draft)
He wants to live.
Solkin taps his finger against the rune, drawn in blood on the end of the branch. His blood. All he gets are sparkles, not orange, but dark green. That is the price of dark magic.
He must hurry or perish. The creature that hunts him is foul and vicious. It will catch up to him.
He tries to draw fire into existence again. This time some of the branch is charred, but the flames do not hold.
Why did he not pay better attention in class? He dashes on, fumbling with his pocketknife, across the field, towards the tower, its top lit like a beacon, upon a nearby hill. It may as well be across the lake.
Something growls behind him. He turns, and in the night, three eyes rise and fall with every step the fellhound takes towards him.
Work! he thinks and slashes his palm, drenching the branch in his blood, as he summons fire from the depths of his soul.
The green flame consumes the branch, and the fellhound halts its approach, its three eyes watching, but it does not turn to run.
Solkin holds out the burning branch, then watches as the flame creeps down towards his hand.
No, no, no!
He burns, and the fellhound howls, and Solkin watches as it approaches, and begins to chew at his charred belly.
Solkin wails, wishing he were dead.