So, whats the difference between level and xHit-Dices. I mean, I was checking the spell descriptions, and I bumped into Death Spell, it has no saving throws but it does not work in creatures with more than 8 hit dice. Just fine, so I was ttacked by a party of 4 mind flayers and 3 umber hulks. I pulled Edwin back and my archer, sent my fighters ahead to work as cannon fodder, and thought wich spell should Ed use. I saw death spells and decided it would do just fine against the umber hulks, so it would be easier to nail the flayers without those evil green hulks. I cast it and everyone, everyone I say, died. I just thought to myself "holy f***, if I knew that when I was looking for the Hammer of Thunderbolts."
Someone once told me that xHit-Dice is roughlly a Level. Is it? If it is how can I tell if a creature is low enough to have death spell working on them?

Each time you level up, you gain a hitdice worth of HP. So in theory, a 2 hitdice monster is level 2. However, this is not always the case, especially when it comes to Dragons.

For game purposes however, take it as all creatures level 8 and below(plus all summons). Remember though, Magic Resistance will block Death Spell(but it will ignore summons' magic resistances).

Aye, thanks a lot Dundee.