The ideas look cool after a quick peek, but I think they are understatted. When I arrive at home I'll edit this with better explanations
EDIT: Well, let's see:
First of all, I'm not trying to be harsh, I like your cards, but I want to give you my opinion. (Also I can practice my english, haha)
The Grave Guardian: A Agithore said, the Discover keyword only words in cards of your own class. That could be fixed with something like "this card can discover cards of other classes" or something like that to make it "unique". But still, is a 4/8 for 10 mana. Understatted.
The Greedy Knight: Again, understatted even with his effect, needing 4 damaged minions to be efficient. Although, if you tweak the base stats or even the buffs can be pretty interesting combined with Whirlwind.
Shadow Warrior: Cool idea, and, while I see it properly costed, I think 4/8 or even 5/7 could be a better distribution. I really like this effect, to be honest.
Elder Phantom: My favourite concept out of all cards you made. I think it would be better with a 6/4 than a 4/6, taking more advantage from the Divine Shield, but that is my opinion and your card is pretty cool too. (Also, insane synergies with some of the new paladin cards from KFT)
Grave Advocator: WELL, this is basically a 5/10 with a great buff for every undead. I call OP.
Lord Valdaros: Understatted, but interesting effect. I would like what synergies you have in mind with this card.
Angos Creature: Cluelesspug already said it: five charge minions can be VERY devastating with some druid buffs.
Enigma Sorcerer: What do you think if I said it could be better as a Battlecry than a Deathrattle? That would made up its low statline.
Queen Lyra Northling: The worst by far IMHO. The effect just isn't worth the 11 mana. Maybe something like "Battlecry: for the rest of the game, your secrets cost (0)" or something like that.
Black Hologuard: As cluelesspug said, It has zero impact in board, and while can be useful on the long run, the stats are too low.
I would like what synergies you have in mind with Undead, that would explain some decisions, I guess.
Angos Creature InnervateSavage Roar is a 3 card, 26 damage combo that also gives you a huge board. Even without Savage Roar it's 12 burst damage AND a full board. This card is quite overpowered.
My favorite design is Elder Phantom. He has very good stats and an effect that makes him hard to kill. As Draziken said, all of the other cards except for Grave Advocator (because he buffs himself) have really bad stats. For 8-10 mana, you should be getting huge piles of stats in addition to their effects.
As an example, Black Hologuard is essentially a 9 mana 5/5 that does nothing the turn you play him. He is completely unplayable with 5/5 stats. Another example is The Greedy Knight. For him to be worth playing, he'd probably have to be a 12/12. That means having five damaged undead on the board, which is probably a very rare situation, even in Warrior.
I really love your presentation but think you need to flesh out your ideas a bit more. Many of these Legendaries are too simplistic and/or bland and aren't really interesting, while also being understated and weak.
The Grave Guardian discovers cards Undead Legendary class cards - but since you can only discover cards from your own class, none of the classes will have a choice of 3 to choose from. Seems pointless.
Very nice editing and presentation, but as said above, the effects are too simplistic for legendaries and the wording could be improved ("Your undead minions can be treated as abeast minions" ; "Gain stealth whenever you play an Undead minion" ...).