New Game Plus is a gameplay mechanic in Demon's Souls.
New Game+, or NG+ is entered once the player defeats Old King Allant and chooses to either kill The Maiden in Black or let her lull the Old One back to its slumber.
The story and events in New Game+ are identical to New Game. The primary difference is an increase in game difficulty, in the form of more powerful enemies, and greater reward in souls for their defeat. The player keeps all non-story based acquired weapons, armor, items and rings. Key items will be lost, for you to rediscover.
All key items.
New Game+ and BeyondEdit
Once the player reaches the end again they will start with an even harder difficulty and an increase in souls but only marginally so. The biggest spike is NG to NG+ in which many enemies gain 2, 3, or 4 times the damage and souls while health usually only doubles. After this, the increase is minute. The difficulty increase stops at New Game+7.