Not from work

IIRC it's a warlock power, of level... errgg... 6-8? I'll get back to you on that...
Going to assume you mean the level 5 daily Curse of the Bloody fangs, hit is damage, miss half damage, sustain is the target and any enemy adjacent takes 1d10 (save ends)
1 First round you cast it and hit, target takes damage, if you miss target takes half of that damage.
2 On the targets turn it gets to make a save. It doesn't take any damage or complications from having the effect on him during it's turn, it just gets to save at the end of it's turn to end your effect. IF it saves go to step 5, if it fails step 3.
3 Back to your turn, you now get to choose to sustain the power or not, if you sustain the target and all adjacent enemies take 1d10 and on to step 4. If you do not sustain it is finished and no longer sustainable so you go to step 5
4 On to the enemies turn, they get their save at the end of their turn to end the power again, if they save on to step 5. If they fail their save you go back to step 3.
5 Power has ended, no more sustains or saves
There is no wording in sustain descriptions (pg 59,278 phb) or the power description (pg 134 phb) that indicate you need to hit to sustain anything, just in case anyone mentions that.