A unique idea would be having a certain boss spawn every Wednesday at a set time (I suggest we use the Hati wolf from RS -
click here and Sköll -
click here) - These two bosses would be on a rotation so Hati would spawn one week, and then Sköll would spawn the week after and so on...
The boss as you said would have a large amount of health therefore you would need quite a few players on the server to help you kill it, but it would not do major damage to you. The boss could drop the items it drops on RS (minus the Eir items) and they would provide additional XP however they would degrade quite quickly. (1/3 of brawling gloves?).
When the boss spawns it could send a message to all players something along the lines of "You hear a
frightful howl in the distance". (
frightful would change depending on where the player was located when the boss spawns e.g. "You hear a
dull howl in the distance")
EDIT 1: Added Sköll to the Wednesday rotation.
EDIT 2: I forgot to mention that these bosses should be randomly spawned like the fallen star, however should not be located near any populated area (mainly for those who do not wish to participate and those who are low level).