After completing both the Sun Stone and Rainbow-Color Shell sidequests, talk to Bosch in the basement again.

Bosch: Oh! The Sun Stone?
I'll try crossing it with this one right away!

The screen fades out, there are noises as of a busy construction site, and then the scene fades back in.

Bosch: It's done.
Take this with you!

Bosch: Huhu.
Don't think this means I'm done.
Bosch is really enthusiastic this time!

The screen fades out, there are noises as of a busy construction site, and then the scene fades back in.

Bosch: It's done.
Take this with you!

The Rainbow is Crono's best sword. See the items page for details on both items.

Bonus rewards for clearing both quests: Rainbow Glasses, Rainbow

The Black Dream is accesible in B.C.12000, A.D.600, and A.D.1000. The era makes no difference as far as what happens inside, except for one reference to the year. Black Dream plays inside, making the hi-tech environment seem creepy, and continues playing during some (but not all) battles.

There are lots of items in here, so here's a link to the items page for details.

Zeal awaits their arrival just inside.

わらわ, as mentioned before, is a rare humble female version of "I". I find it appropriate in this situation that the kanji for it, 妾, also means "concubine".

Is that the "royal we" or just poor translation there?

Zeal gestures, one of those little portals Dalton liked using opens, and a big mutant something comes out. Boss Battle 1 starts playing, so if there was any doubt about why she summoned it....

Zeal vanishes, and the battle commenses.

The Mega Mutant actually has two parts, a torso and some kind of mutated "foot". The creature likes to use status attacks, often against the enitre party, so equip protection. Other than that, it's fairly unremarkable. Ayla can steal an Elixir from the torso, and a Safeguard Hat from the base.

With the boss down, the party can proceed. In a few rooms, the Walls themselves attack, but they're more than a mere nuisance because Ayla can steal Speed Capsules from them. Unfortunately, the walls, once beaten, stay dead, so you can't keep fighting them until everyone's speed is maxed out. Some rooms have eyeball monsters that, like the earlier boulder monsters, lock skills, dodge often, and eventually run away, but the little ones give 100 tech points each if killed. One of the chests further in has a Magic Seal, and a room later on has a save point, a Safeguard Hat, a Nova Armor, a Haste Helm, a few Last Elixirs, and two Nu. One sells supplies, and the other....

Lackey: Sheesh! How long have we
got to listen to what he says, ribbit?

Two frogs push in what look like the seats from the Sylbird.

The seats blast off and the party lands back outside.

In the next room, there's a swirling something that summons a big monster that looks like a gold Hekeran. It hits back, hard, whenever attacked, but only attacks when hit. Ayla can steal a Power Capsule from it, and it drops Elixirs. Since going back to the previous room makes it come back, this is an unlimited source of both if you have the patience. There's a Stardust Cape in a chest not much later, a Power Seal and Speed Capsule a few rooms after that, and another Speed Capsule in the next room. A few rooms later has monsters appearing out of nowhere; Ayla can steal Magic Capsules from the reptilian ones with the mouth tentacles, but once killed, they're gone permanently. The same room has a Speed Capsule and a Last Elixir in chests.

After the next save point, the floor goes funny and the Giga Mutant shows up to do battle. This one is virtually immune to physical attacks, and the torso counterattacks by draining the MP you need to hurt it. Other than that, it's unremarkable. Ayla can steal a Barrier Ring from the torso and a Hit Ring from the base. A later room has more attacking Walls for Ayla to pillage, and another Speed Capsule in a chest.

On the far side of the room, Tera Mutant appears. Like the Giga Mutant, it's virtually immune to physical attacks, but to make things worse, the base counters physical attacks with a strike that lowers HP to 1, has extremely high magic defense, and absorbs all elements! Don't worry, just keep attacking the torso with magic. It will literally suck the life out of the base. You can probably inflict damage fast enough to kill the torso off anyway, and if not, it will eventually suck the base to death. If either part dies, the other part dies automatically soon after. Ayla can steal a Muscle Ring from the torso and a Power Seal from the base. The chests nearby contain a Last Elixir and the White Stone.

Just after the next corridor, one of those mini-Lavos creatures pops out. This one is a bit stronger than the ones on Death Mountain, but the same strategy applies. Ayla can steal a Haste Helm from the head, and a Protect Helm from the shell (which WILL counterattack painfully, but it's probably worth it).

The next room has five Walls for Ayla to loot Speed Capsules from. The room after the Walls is where things get interesting. Lining the pathway are what appear to be the six playable characters other than Magus in stasis. At the end of the corridor, Zeal appears and Zeal Palace starts to play.

Queen: Kukuku....
What sleeps there is your future....
The dreams that might have
come true after this....
The joys and sorrows you might
have been able to earn....
Your tomorrow itself!

Queen: This Black Dream flows across
all temporal dimensions....
While awaiting that time when
Lavos-sama will awaken....
Your future will invariably reach
here eventually.
You have no future!
Not as long as you don't beat me
and stop this Black Dream!

The party surrounds her, praparing to do battle.

Queen: Come, children of humans!
I shall lure you into the midst of
Lavos-sama's slumber....
To an eternal black dream...!

Time to fight. Zeal Palace keeps playing during the battle. Zeal nearly always attacks with ハレーション (Hallation), which drops each character's HP to 1, but she'll hardly ever actually finish them off. Ayla can steal two Last Elixirs from her. Upon losing, Zeal drops another Last Elixir.

Queen: I can't show my strength here....
I've thought of something good.
I'll stick you vermin into the Demonic Vessel.
I hope you're thankful....
To be able to become a part of this ship!
To be able to become a part of me!!
To be able to become a part of Lavos-sama!!!

She magics them into a distorted dimension where the Demonic Vessel is, and the boss music starts up. There are a lot of messages that can pop up in this battle, so I'll just list them all here:

In short, it goes through an active phase (indicated by the first message) during which it raises defense power when attacked physically except by the Grandleon, raises attack power when attacked magically, and heals Frog (in proportion to its own attack power) when he hits it with the Grandleon. At the end of this phase, it unleashes an attack on the entire party, which starts out weak but can be deadly if the attack power rose too much. Then it stops messing with energy for a while, reverting to its original attack power and defense power some time during this phase. Eventually, it returns to the active state.

For best results, attack only with the Grandleon when it's active, and let loose with all you've got when it isn't. Ayla can steal a Last Elixir (note that stealing attempts are considered magic attacks).

Once it goes down, the party appears on a surface that looks like it's on the outside of the Black Dream. Zeal warps in.

Magus: Foolish....
All existence is unable to escape
from the fate of perishing....

Magus: Zeal.
A sad woman enthralled by Lavos.
Simply pitiful....

Magus: I will bring it all to an end
by my hand!

Zeal morphs into a large mask and two huge gloves, and the battle begins. Whichever music is playing continues playing for the entire battle. The glove on the left of the screen counters all attacks against it by dropping the offender's HP to 1, the one on the right counters by stealing all of that person's MP, and both are nearly invulnerable anyway. The head will occasionally use Hallation, so be ready to heal at any time.

Ayla can, and should, steal a Prism Dress from the glove on the left, a Prism Helm from the one on the right, and a Last Elixir from the head, but be careful since the gloves will counterattack. And bring Magus for the music!

Once the mask dies, it's all over, and Zeal reappears in her normal form.

She does something, the Black Dream shoots energy downward, and there seems to be a response of some kind.

The reaction from the ground increases, the Black Dream decends and disintegrates, and Lavos breaks through from the depths.

Just so you know, here's a list of possible spoils for passing through the Black Dream, not counting miscellaneous healing and other items: 2 Safeguard Hats, Magic Seal, Nova Armor, 2 Haste Helms, Stardust Cape, 2 Power Seals, Barrier Ring, Hit Ring, Muscle Ring, White Stone, Protect Helm, Prism Helm, Prism Dress, a bunch of Last Elixirs, 4 Magic Capsules, 17 Speed Capsules, and as many Power Capsules as you have the patience for Ayla to steal. Obviously, that's a lot of stuff. I'm fairly certian that the Black Dream sidequest is also required to unlock New Game+.

To make things even more interesting, the Black Dream remains present in the eras before you cleared it, so you can fight Zeal twice more and steal multiple Last Elixirs, two more Prism Dresses, and two more Prism Helms! However, any chests you opened in the later eras will somehow be open in the earlier ones, and any one-time battles fought are nowhere to be seen, with the exception of Zeal herself. Even Lavos is already broken open at the end.