The void summon dragons are creatures that have an effect after they are summoned from your void. The combo involves using a card to get the dragons into the void, and another card to reanimate them to trigger their effect.
These dragons gain effects after being reanimated. Most of them are expensive and not worth playing without using their void summon effects. On their own, they're not much better than basic creatures. For example, Voidjaw Dragon is a 7-mana 8/6 creature, weaker than Burnished Bull, a 7-mana 10/10 that's never seen play. Your competition is other 7 mana creatures that don't need any setup, like Demogorgon, Ember Oni, and Thaeriel, The Fallen. So you have to use their void summon effect for the dragons to be worth anything.
The payoffs of reanimating them range from mediocre, such as Shovelskull gaining 3hp, to game-winning with Lokhart clearing the board and dealing tons of overkill damage.
There are a couple of different ways to get your dragons into your void. You can play the dragons out normally and have them die but you need the mana to play them while avoiding transform/obliterate/soulless that keeps them out of the void.
Contagious Ghoul or Corrupt Ceremony can pull a dragon from your deck into the void it's on top of the deck. Alternatively, you can guarantee pulling creatures into your void with Tempest Altar for 6 mana.
Dragonskull Charm is not random, but you need to unlock enough mana to play the dragon, making it slow and vulnerable to relic removal.
Pirate Bully is semi-random, letting you play out the creatures you don't want to discard. However, you need to run a few creatures in your deck to consistently discard your dragons.
Reanimation cards summon your dragons from the void.
Fated Demise and Vasek's Awakening summon the top (rightmost or most recent) dragon of your void, so you have to pay attention to the void and be careful playing more dragons so they don't cover up your intended reanimation target.
Fated Demise destroys the dragon at the end of the turn, so you use it mostly to trigger the void summon effect once. On the other hand, Vasek's Awakening lets you keep the dragon around. However, you aren't likely to win from just having 1 big dragon around since there is a lot of hard removals available.
Raise Dead lets you choose your target but it is slow at 5 mana.
Tempest Altar summons 5 dragons, but you need to activate manasurge 9 by spending 9 mana in one turn, either by reaching 9 mana naturally or using Necronomics to refresh 5 mana.
Draka Omenbinder needs manasurge 6 to reanimate. Also, it makes your Fated Demise and Vasek's Awakening worse since your opponent can kill it and cover your reanimation target.
To make the combo work, you need all three parts: the dragon, a way to get it into the void, and a way to reanimate it. Without all these parts, the cards won't have a big effect. If you're putting in the effort to activate the void summon effect, it makes sense to aim for the biggest payoff, as the effort of getting the dragon into the void and reanimating it remains the same regardless of the summoned creature. But it also means that your reanimation cards are stuck in your hand more often as you wait to discard Lokhart. Deepwater Whelp helps you find Lokhart but you have to be careful it doesn't end up covering Lokhart.
Overall, the combo is challenging to pull off, with the payoff not justifying the effort outside of Lokhart. Moreover, it is vulnerable to void removal like Gleamweaver found in the sanctum.
Taking what I've talked about here into account, the optimal deck would run only Lokhart as the payoff since it is the strongest effect from being summoned from the void, Pirate Bully for getting Lokhart into the void as it is semi-reliable knowing what creatures in hand you have for it to discard and being faster than Altar or Charm, and Fated Demise since it's the cheapest reanimation spell and you don't need Lokhart to stick on board.
You can see this deck by [8M] Hpain where he puts the Lokhart Combo into a Bitter Endings deck which gives the deck a combo to clear board/kill the opponent.
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