First, the undatakah combo requires four cards. That can be quite a lot, especially since you need it fairly quickly to beat most aggro or zoo decks. To do this you need card draw. Unfortunatly, card draw cannot come from deathrattles as that will ruin the undatakah combo. This is why you use the novice engineer and acolyte of pain. Spikeridged is important because you can give the undatakah taunt. This allows you have an immortal taunt, haulting damage that your opponent can deal to your hero.
In this deck the goal is to play immortal prelate and silver vanguard ( while undatakah is in your hand) and then any other third deathrattle card, (cairne Bloodhoof or tirion fordring) then you can play undatakah. If his battle cry goes in the right order with no other 8 costs in your deck, he will recruit himself and really become immortal. Even if it goes in the wrong order, when you draw him again and play him it will certainly work on that try. Then you can play spikeridged steed on it and it is an immortal taunt that your opponent can never remove unless there is a silence mechanic card.