You should have read about Bellem Ruins from v2.4 New Feature: Island of Fire article:
"Bellem’s Ruins have been discovered on the Island of Fire. You can enter Bellem Ruins from Bellem Tombstone found at ‘Island of Fire’ (Channel 1 only). You must have a Key of Bellem and can only enter within a specific time period. There will be a server wide announcement whenever the Bellem Ruins are open. Bellem Ruins is a place where mini bosses and bosses are summoned randomly. After hunting the Mini boss and the boss monster, you will find a treasure chest with your name on it which can only be opened by you."
How can I enter Bellem Ruins?
You can enter Bellem Ruins when the Bellem Tombstone is unsealed. Every 3 hours, there will be a server-wide notice:
[Notice] After 10 minutes, the Bellem Tombstone will be released from its seal.
Another notice will flash after 10 minutes:
[Notice] Bellem Tombstone is unsealed.
By clicking on the unsealed tombstone, you can enter Bellem Ruins if you have a Key of Bellem in your inventory. The tombstone remains open for 10 minutes. After 3 hours, the cycle repeats with the notice of the tombstone re-opening in 10 minutes. This means there is a time gap of about 3 hours 20 minutes between each opening of the tombstone.
Clicking on the sealed tombstone will provide you with the background story:
"Montoro... He attacked my land, performed brutal experiments on our people. All my followers have become monsters because of him. I have to seal this place to prevent them from escaping and harming others. After using the key on the box, I shall destroy the box and hide its pieces along with the key. This way, it will give you a hard time in searching for the key. Even if you have the key, you might not be able to defeat them. Once you fail, you will have to return to the place where you came from. This sealed entrance will be opened once every 3 hours."
Unlike Underground Relics which you can sneak in without a key (by waiting for someone else to open it), each family entering Bellem Ruins must have a key. This is not a quest instance, so everyone who enter the ruins will be on the same map.
Where can I find Bellem Tombstone?
Bellem Tombstone is located at G11 on Island of Fire (channel 1 only). Since Bellem Ruins is located in Island of Fire, all families are on PvP mode for PvP servers. (No PvP mode for non-PvP servers, just like Island of Fire.)
Where can I find the Key of Bellem?
This key is a random reward for collecting 20 Cabinet Pieces for the Old Woman in Auch (D4). Once you enter the ruins, the key is consumed. For more information, see Old Woman & Cabinet Pieces blog entry.
How long can I stay in the ruins?
You can stay until 1 hour has passed since the bosses spawned. Like Underground Relics, you may not re-enter if you leave the dungeon. Like Island of Fire, you may not change channel or save a warp point within the ruins.
What is the Bellem Ruins map like?
The map looks exactly like the map of Underground Relics. Note that unlike Underground Relics which brings you to a safe platform first before "teleporting" you into the skeletal mobs, you immediately enter into the open space with mobs when you enter Bellem Ruins. Grids D-E are free from mobs, so you might want to lure the bosses over here to fight them.
What kind of monsters are inside the ruins?
Failed Subjects (Lv.81 Undead) are the non-boss monsters inside the ruins. These monsters drop pure ores and mega ores. When killed, they respawn in groups after a few seconds, but once the bosses spawn, they do not respawn anymore.
About 10 minutes after the tombstone is unsealed, a zone-wide notice will flash at the top of the screen:
BOSSNAME1 Summoned Monster.
After 1 hour, all families will be moved out of the zone.
BOSSNAME2 Summoned Monster. etc.
There could be 1-5 random bosses (Lv.90-100) spawning at the same time in the middle of the map. They do not drop anything when killed. If you do not defeat the bosses within an hour, there will be a zone-wide notice:
After 1 minute, all families will be moved out of the zone.
If you still do not defeat the bosses, you will be moved back to Island of Fire.
Known bosses include Gerero, Swamp Frogfish, Dr. Fran Mothtein, Dream Blade Antelope, King of Greed, Elmorc, Phobitan Chieftain, Phobitan General, Sabel Tiger, Victor, Cerbera, Castor, Pollux, Wahlansche, Captain Sharffenberger, Giant Cockatrice, Gullfaxi, Sunflower Dandelion Gorilla, Thoracotomy, Fallen Champion, and Goldhorn Demon.
My worst boss combo so far was Gerero x2, King of Greed, Dream Blade Antelope, and Thoracotomy. (It is somewhat pointless to post boss stats, since the boss types and levels are random.)
These bosses do not count towards quests and missions, so killing Swamp Frogfish here will not help you advance Bahamar Pioneering Quests.
What do I do with my Family Treasure Box?
Each family that enters the ruins has his own Family Treasure Box, which remains locked. Once all the bosses are dead, there will be a zone-wide notice:
You can open the treasure box.
All families will be moved out of the zone in 90 seconds.
Other families (if any) may not open your Family Treasure Box. The box contains a Lv.84 elite weapon/armor (AR/DR 26) or Lv.84-88 summon tool (e.g. controller, hammer, pendant). The drop is tagged, so other families may not ninja-steal your loot.
What is Bellem's Treasure Box?
Once the family treasure box has been opened, there will be a zone-wide notice:
Bellem treasure box has been summoned.
A huge treasure box will appear near the middle of the map. All families in the ruins may then attack the treasure box.
Upon destruction, Bellem's Treasure Box drops a Lv.84 elite weapon/armor (AR/DR 26), Lv.84-88 summon tool, or rarely Lv.100 weapon (AR 30; e.g. Conqueror Cannon, Bracelet of Ziz). There will be another zone-wide notice:
All families will be moved out of the zone in 10 seconds.
So quickly loot before you are moved back to Bellem Tombstone in the Island of Fire. This drop is also tagged, so other families may not ninja-steal your loot.
This blog entry has been reproduced as [Guide] Bellem Ruins Revealed in GE IAHGames forum.