Activating the Grand 
Once you have both halves of Elden Ring Items the Dectus Medallion, fast travel back to the Grand Lift of the Dectus Site of Grace. At the lift, stand on the circle between the two statues and use the Medallion. This will activate the Grand Lift, allowing you to ascend and be transported to the Altus Plateau. From here, you'll have access to new areas and quests, including Leyndell, Royal Capital, which is crucial for the game's progression.
What Happens After Activating the Grand Lift of Dectus?
Activating the Grand Lift of Dectus will not only open up access to the Altus Plateau but also trigger the Radahn Festival at Redmane Castle in Caelid. This festival is a key event where players can challenge Starscourge Radahn, one of the game's major bosses. During the festival, access to Redmane Castle is more limited, and several NPCs will replace the usual enemies. Notably, the bosses, Crucible Knight and Misbegotten Warrior, won't be present, making them optional to defeat if encountered during the festival.
What to Know About the Radahn Festival: 
The Radahn Festival can be triggered not just by activating the Grand Lift of Dectus, but also by progressing through Ranni's questline or by simply visiting Altus Plateau. If you visit Redmane Castle after the festival, you'll be able to explore the area more freely and obtain various items, including weapons and Smithing Stones.
Alternative Route: 
The Ruin Strewn Precipice
For players who find the enemies in Bellum Highway too challenging, there is an alternate route into the Altus Plateau. The Ruin Strewn Precipice is a hidden path that players can take to bypass some of the tougher enemies near the Grand Lift of Dectus.
How to Access: 
Start at the Ravine Site of Grace in Liurnia of the Lakes, and follow the path leading to the Ravine-Vailed Village. From here, take a ladder up to enter the Miners' Cave. Proceed through the cave and take an elevator to the surface.
Getting to the Altus Plateau: 
Once outside, you'll find a Site of Grace and a series of ladders to climb, avoiding or defeating enemies like Giant Bats along the way. After defeating the Magma Wyrm Makar boss in a fog wall fight, you'll emerge into the Altus Plateau, ready to buy Elden Ring Runes continue your adventure.