The Boss Raid monsters are an event that started the summer of 2007. It consisted of four, super-strong monsters--one for each map.
With the release of Strayana a new Boss Raid was added, making five total. The names, levels, and statistics of the Boss Raid monsters are as follows:

	Grand Balrog (This monster has been taken out of the map of Juno) Level: 30 Location: Juno Hitpoints: 24,837 Attack type: Physical short-range, Attack skills: ? Other skills: Teleporting, Grand Trust (like Perfect Body) Monsters spawned when near death: Elder Werewolf (level 27)
	Grand Red Dragon (Fiery Raleigh) Level: 50 Location: Merac Hitpoints: 85,046 Attack type: Short-range physical, magical Attack skills: Bite (physical), Red Breath (magical) Other skills: Teleporting, Grand Trust (like Perfect Body) Monsters spawned when near death: Cerebrii (level 51)
	Grand Devilroad Kamira (Queen Sagira) Level: 70 Location: Dratan Hitpoints: 207,685 Attack type: Magical Attack skills: Kamira Ball (magical), Kamira Meteors (magical) Other skills: Teleporting, Grand Trust (like Perfect Body) Monsters spawned when near death: Death Goddess (level 35) Experience: 10,625,300
	Grand Gold Dragon (Golden Artemis) Level: 95 Location: Egeha Hitpoints: 906,749 Attack type: Short-range physical, magical Attack skills: Bite (physical), Gold Breath (magical) Other skills: Teleporting, Grand Trust (like Perfect Body) Monsters spawned when near death: Cursed Weaver (level 88)

If you have ever run into one of these monsters, you will know that they are very big and very strong. Each of them has a lot of HP, uses long-range AoE attacks, and is extremely difficult to solo (kill alone). These monsters were implemented to encourage players to get into parties with each other and work together.

Due to the addition of the Evasion patch (March 04/08), evading magical monsters is far easier now. With plenty of Magic Resistance (which is magical evasion), you can dodge even these monsters' attack skills. This makes soloing all of them possible, although it takes quite some time.

The Boss Raid monsters respawn 2-3 hours after being killed. If a Boss Raid monster is not touched (not attacked) for 14 minutes after arriving somewhere, it will teleport to a new spot. The Boss Raid monsters usually end up spawning either in the most active spot on the map or where the lowest level player is. This is because the Boss Raid monsters are coded to spawn where the latest kill was made. Low levels kill much faster and easier than high levels, so their chances of being spawned on are greater. Anyone who is doing a lot of killing in one spot has an increased chance of being spawned on.

The Grand Balrog has been deactivated in Juno and only spawns when a Game Master summons it. The reason for this is that having a Boss Raid in Juno would cause mid-high and high level players to hang out there and spawncamp it, resulting in more harassment for lower level players. This information was taken from this post.

The Boss Raid monsters are the only way to get unique ("rare") accessories. Here's a list of the different unique accessories that can be dropped by these Boss Raid monsters:

	Crest of Rainbow Physical Prowess: 12 Swiftness of Reflexes: 12 Power of the Mind: 12 Physical Fortitude: 6				Eyes of Spirit Long Range Physical Hit Rate: 168 Close Range Evasion: 78 HP: 44 MP: 44
	Tears of Knight Physical Prowess: 12 Close Range Physical Attack: 16 Close Range Physical Hit Rate: 168				Rage of Demon Power of the Mind: 12 Magical Attack: 18 MP: 100
	Chain of Tiger Physical Prowess: 16 Close Range Physical Attack: 8				Eye of Wise Men Power of the Mind: 16 MP: 100
	Stone of Shadow Swiftness of Reflexes: 12 Close Range Evasion: 112 Long Range Evasion: 93				Origin of Land Physical Fortitude: 6 All Types of Physical Defense: 12 HP: 100
	Pierce of Wind Swiftness of Reflexes: 16 Close Range Evasion: 78				Stone of Life Physical Fortitude: 16 HP: 100

When you kill a Boss Raid monster, it drops many things. Among these many things can be Heaven Stones, Tool Aids, Drake Eggs/Pan Flutes, various magical accessories, Medals of Honor, and possibly a unique accessory. If you get a unique accessory and pick it up, it will have a name but no stats. The Identifying seals page explains how to make the stats of unique accessories appear.

Most of this information was taken from here.