GRealm is a modification to Terraria from Grox the Great which adds a variety of content, including new NPCs, Projectiles, Items, Tiles, etc. The content is spread all across the tiers (and in some cases, creates tiers of it's own), and will be encountered throughout the game.
Much of the mod's content revolves around a scaling event called The Horde. With each tier of progression, its monsters become more and more varied and deadly. It yields a multitude of powerful and unique weapons, armors and accessories to find and craft.