DMing a Homebrew Campaign. The party is about to face an Obsidian Dragon. I want to make it a very unique creature. I have some things nailed down that I want, but I need a bit of help making it more unique. So far I have it having high AC and Fort, low Reflex. It's attacks will deal ongoing untyped damage ( a sort of bleed damage). One of its abilities summons an obsidian spike from under a PC (area burst 1 within X) that leaves aline of sight blocking spikes in its wake. The central spot is impassable, the adjacent squares at difficult terrain and deal damage to anyone passing through. That's all I've got, but it's a VERY important fight, so I want it to be very unique. Any other suggestions?