Umm, the idea itself is awesome.. But.. You know, I always felt that minecraft has no risks.. I mean it.. What? The wither can destroy some blocks? The enderdragon is stuck in another dimension? This has the potential to be fear, instead of a boss.. I would like to see this mod going that path.. And as Reika said, the fight is only secondary.. So.. what if the monster has a low chance of attacking you? He should always follow you, ALWAYS. maybe 20 to 30 blocks away.. But make his presence obvious.. So basically, you feel it all the game when caving until one day bedrock breaks.. But in that case it shouldnt just come out and kick the crap out of you.. I think one trait this monster should have is to be able to pass through any block except bedrock... So you cant be safe even in your base, and maybe every minute it has a 1 in 20 chance to engage, so you cant foresee when it will attack, the best part of fear is doubt.. If you can just fight it, then you're basically done, but if you dont know if it will atack when you're close to that lava pool or fighting something else, then you feel true fear..
Also, its a semi corporeal thing, so why not divide? You kill it then it divides and you have to kill it again.. and again.. and again.. and when you kill the little ones.. It could just become myst and yet again.. 1 in 20 chance to engage again in corporeal form
I really want something to scare the crap out of me and make me think twice before going out unprepared...