[Dread knight command ability]

During which phase can I use the dread knight command ability with my vampire lord on zombie dragon ?

[Pile in question]

Lets say I have a hero like Arkhan and he attacks the unit closest to him and kills the unit with his first attack. Arkhan has 3 more attacks and another enemy unit is 2 inches away from Arkhan. Can I pile in with Arkhan and attack with my remaining 3 attacks ?

[Artifact weapon question]

When an artifact weapon says "pick one of the bearers melee weapons to be...." do I have to pick only the weapons that my hero has or can I pick one of the heroes mounts attacks?

[Vampire lord question]

The GW store offers 5 different vampire lords for sale that all share the same warscroll, what are the stats for each one? I assume the one with the skeleton horse gets the "nightmare steed" ability. What about the other 4, which ones have "flying horror" and which ones have the "chalice of blood"?

[Endless spells questions]

Does arkhans staff modifier +2 to cast and +2 to unbind apply to endless spells.

Also does his command ability "first of the mortarchs" also increase the range of the endless spells that I cast.

[Line of sight for wizards]

When determining if a unit or model is in line of sight for a wizard to cast a spell on. Do you measure from the highest point of the wizard model, for example the tip of  Arkhans staff, or eye level of that wizard model?

[Arkhan skeleton guard battalion]

The skeleton horde start collecting box comes with a warscroll battalion "arkhans skeleton guard". Arkhan, a unit of black knights and a unit of skeletons. Is this battalion legal in matched play and how many points does it cost?

[Skeleton units of 20 or 30 question]

I hate assembling them but looks like in order to get the most out of them I'll have assemble 10 more, thanks.

[Skeleton units of 20 or 30 question]

If I use a unit of 20 skeletons, that means that the attack characteristic for each model is 2. What if 1 of those skeletons dies and now im down to 19 skeletons, does their attack characteristic drop to 1 until I revive the 1 dead skeleton or will it stay at 2 because the unit started as a unit of 20 skeletons?