“Climb to Vostok Observatory.”
Look for the break between the rocks in front of you. This will give you the path forward. Start along it.
You quickly reach a break in the path. Just jump over it and climb from there to the ledge above.
Follow the bend at the top of the ledge over to the right and then forward as you continue to climb upwards on Felwinter Peak. Go around the corner on the narrow ledge ahead. You will find Fallen waiting there to attack anyone trying to get up and defend the Observatory. You are likely to encounter Dregs, Shanks and a Captain.
Head on up the slope that the Fallen were on and then press on forward. Go across the narrow peak and continue on forward toward the observatory and the top of the mountain.