UPDATE: As of July 2022 most, it not all, versions of the Raid Offer have been changed to requiring you to open 3 Sacred shards and requiring a purchase. It may no longer possible to do this offer Free-to-Play.
The offer is “Acquire and use 2 Void shards in 10 days.” This game shows up on all of the offer walls with varying rewards. I am doing it during the RevU Holiday’s: Triple Bonus Event. This can be done easily if you are open to spending some money in order to make some money. There are 3 methods to completing this offer and I will detail them starting with the fastest method.
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So first up is the Pay to Win method. Here you will level up your Lord to 10 or 15, at which points you will be offered a “limited time” sale that includes 2 Void shards. The level 10 pack cost’s $14.99 and the level 15 pack is $9.99. So depending on how much the game is being offered at ($45 in my case) you can earn 30-35 bucks by making an early purchase.

The second method does not require a purchase. If you complete the Campaign on Normal all the way through earning 1 Star you will receive a Void shard for the completion. There are 12 chapters in all and you will need to level up your heroes accordingly so that they can make it all the way through. After that you then can get a second Void shard by completing the whole Campaign with 3 Stars. This requires that you use only 2 Heroes the whole way through bosses and all. You want your starter Hero to be Kael and his comrade will be Warpriest whom you get during the early Campaign missions. Kael should be ranked up to 5 Stars and Warpriest to 4 Stars. Ascension of heroes is optional, but handy. Make sure to gear your two fighters properly: Initially Kael should have 2 pieces of each Life/Offense/Defense. Later Kael gets 2 pieces of Immortal set from Campaign and 4 from the “Daily Login” Life Steal set. This keeps him healed up 3% each turn and 30% for each attack. For Warpriest don’t worry so much as she is tough, 2 pieces of each Life/Offense/Defense and she is good to go.

Lastly the third option for completing the offer is by referrals. If you refer someone and they make it to level 30, you will receive a Void shard. You can refer up to 3 people: friends, family, or more likely yourself. Yes if you have a spare phone or other device in the house you can refer your other email account and grind out 30 levels in 4-5 days. This means multi-tasking to some degree and could get boring fast. It’s likely also confusing having 3 phones and trying to remember which hero is on which phone and all that.
So you have 3 ways to go to finish this offer in the 10 days allotted. Easy right?