Those who have completed Spirit of Summer will know that their work is half done. The Spirit Beast has begun to tear through the fabric of reality and threatens to lay waste to RuneScape. Summer and her family can't stop it alone, so you're needed to confront this powerful enemy.

Brave warriors who are courageous (or foolhardy) enough to take on such a challenge will be required to venture once more into the Wilderness, crossing into the Spirit Beast's realm. Here, you'll be the main thrust of a devastating attack on the beast. If you hope to survive, you'll need the Amulet of Ghostspeak, help from the spirit family, and plenty of food and run energy...

Summary

Where to start the Summer's End quest:

Talk to Summer in the farm ruins, north-west of the Clan Wars challenge hall.
Requirements to complete the Summer's End quest:

Level 23 Summoning
Level 23 Woodcutting
Level 35 Hunter
Level 45 Mining
Level 47 Firemaking
Level 55 Prayer
Spirit of Summer
The ability to evade the blows of a level 700+ beast
A fairly high Hitpoints level and a higher Woodcutting level are an advantage
Access to:

Some interesting new equipment