The Void is the fifth multi-player "dungeon," for level 40-45 characters.
The nasty feature of The Void is the little bats. They attack in groups of 6, and they can stun you. Even a very powerful character can be defeated by those things; you can't attack if you're stunned. So it's best not to try to tackle The Void solo, no matter how strong your attack is.
The boss in The Void is the first version of the floating eyeball that you'll probably encounter. She shows up again as "Fearful Faye" in all Crypt boss levels from level 50 on up, but here's she's pretty much just another boss.
The boss's unique characteristic is that she does her big PATK against the back row. If you have a part with a knight or two as tanks and have average battle ratings for your level, put the knights in back to take the damage from the boss. Most level 40 teams can defeat her.
We did The Void with a 2-toon team consisting of one very strong mage (Brangwen, level 40, BR 38,989) and one middlin' strong knight (Cartwright, level 42, BR 21,956) and got all the way through with no problem at all. Cartwright was not able to defeat even the first bats by himself, but with the addition of Brangwen's mage AOE attacks, the pair were able to breeze right through; no scary moments at all.
Dinah shows up as the Summoner in the void after you taken out a few sets of bats. She is the mother lode of Level 3 Crystaloids, which you will need to make the basic Level 40 gear. In the Void Nightmare, the summoner is also the primary source of Level 40 Legendary Stones, which you need to convert your Level 40 set to Legendary.
The level 40 Summoner wants 4 keys per battle. (It's 5 keys per battle at level 50 and 6 at level 60.)
How to win Summoner battles: Before you call the Summoner, have everybody in the party remove all equipment that adds to defense or hp. That includes astrals. Horses won't make a different but it's easier to see where the treasure chests land if you dismount. Use your rage advantage to do a lot of damage to the mirror party from the very beginning, launching whatever AOE attacks you can to take them down quickly.
If you lose a battle to your mirror selves, don't worry. All you've lost is time. Losing a Summoner battle doesn't cost anything; you don't waste keys or runs unless you leave the "dungeon" before you're done. You only get one multi-player dungeon run a day, so don't waste it. Hit the Summoner any time you have enough keys.
A lot of people just skip the level 40 set entirely, saving their crypt tokens for the Level 50 set. If you like to spend a long time at each level, you'll want this set. If not, you might be better off just getting the 45 set along the way to level 50 and then concentrate on getting the Level 50 set.