At 4th level a gravewalker witch gets both bonethrall (a special ability that acts as command undead but better) and then they have their 4th level patron spell replaced with command undead.
My question is why would you need both? Bonethrall appears to be much better than command undead so I think my house rule would be to keep your 4th level patron spell. Unless the creators were just trying to say bonethrall is so good we're going to give you a spell that won't help you as a patron spell. But in that case they could have just said "this replaces the 4th level patron spell".
I could be wrong but it looks like Command undead (spell) has no HD limit, whereas Bonethrall does (1 HD per caster level).
With bonethrall, intelligent undead get a new save every day. With Command Undead, there is only the save at the very beginning.
So considering that, and the fact the Command Undead doesn't have a hit dice cap, Command Undead is stronger, but bonethrall has no limit to the number of times per day you can use it.
Normally a character has a maximum number of total HD they can control.
Does this ability effectively give the Witch twice the usual HD of controlled undead (one HD pool for each ability)?
Thanks for the comments guys. I see that there are differences (below) but really the powers are so similar that it doesn't seem they would need both of them other than as Doomed Hero says it's so they can have a buttload of undead (more than other casters). If I were the gravewalker I'd rather have bonethrall and more versatility (whatever my patron power would normally be).
Command Undead - 1 save (int), no HD limit, limited duration, 1 creature per casting of spell
Bonethrall - save everyday (int), HD limit, no limit to duration, # of creatures determined by HD limit.
With command undead AND bonethrall you could have 1 temporary powerful undead servant. With CU it guaranteed for a couple days, with BT it depends on the saves (could be just as long).
But with bonethrall you could have a bunch of permanent skeleton/zombie servants.
Note that Necromancer Wizard gets both Command Undead feat and access to command undead at 3rd level and control undead at 7th so there is precedent for multiple ways to order undeads around.