Blackrazor wins ... probably
Ring
If you die while wearing the ring, your soul enters it
Blackrazor
If this necrotic damage reduces you to 0 hit points, Blackrazor devours your soul.
Since being reduced to 0 hit points doesn't usually kill you, then Blackrazor would devour your soul.
PHB, p. 197:
When you drop to 0 hit points, you either die outright or fall Unconscious, as explained in the following sections.
Massive damage can kill you instantly. When damage reduces you to 0 hit points and there is damage remaining, you die if the remaining damage equals or exceeds your hit point maximum.
If damage reduces you to 0 hit points and fails to kill you, you fall Unconscious.
Instant Death
It is unlikely, but technically possible that the necrotic damage from Blackrazor could instantly kill a character. I can think of a few conditions that might make this more likely:
- Low level character might have fewer than 10 maximum hit points
- A character might gain a vulnerability to necrotic damage
- A character may have had his/her hit point maximum reduced by life draining attacks from a wraith or other source
In any case, the key part of the rules regarding dropping to 0 hit points is:
When damage reduces you to 0 hit points and there is damage remaining, you die if...
So, do you drop to 0 hit points, and then die? Does Blackrazor swallow your soul?
Your DM will have to resolve this question. To the best of my knowledge, this is not explicitly outlined in the rules, nor has it been officially answered.
For guidance, your DM might consider this answer found on page 13 or the Sage Advice compendium:
If the damage from disintegrate reduces a half-orc to
0 hit points, can Relentless Endurance prevent the orc
from turning to ash?
If disintegrate reduces you to 0 hit
points, you’re killed outright, as you turn to dust. If you’re a
half-orc, Relentless Endurance can’t save you.
The Relentless Endurance racial feature, PHB p. 41:
When you are reduced to 0 hit points but not killed outright, you can drop to 1 hit point instead.
It is possible that this Sage Advice ruling indicates that even though Relentless Endurance would put you back to 1 HP, the effect of Disintegrate, triggered by being reduced to 0 hit points, is still resolved.
That would imply that, when a character is reduced to 0 hit points, you resolve all of the effects that would happen as a result. In our case, one of those is instant death, and one is from Blackrazor. After resolving both conditions, the character is dead, and his/her soul has been swallowed by Blackrazor.
The effect of the ring cannot not be resolved until after the character is dead. At that point Blackrazor has already swallowed his/her soul.
But in the absence of explicit rules or an official answer, the decision ultimately rests with the DM.
To quote the PHB on "stacking" spells:
Combining Magical Effects
The effects of different spells add together while the durations of those spells overlap. The effects of the same spell cast multiple times don't combine, however. Instead, the most potent effect-such as the highest bonus-from those castings applies while their durations overlap.
For example, if two clerics cast bless on the same target, that character gains the spell's benefit only once; he or she doesn't get to roll two bonus dice.
Basically, this means that the second casting of the Dream spell would be wasted, in RAW.
That said, I don't see why you can't have some fun with it. Looking at how the human mind works, there are rare cases where two halves of the brain can work independently of each other (I have personally experienced this, and it is weird), causing you to either interpret the same thing differently, at the same time, or even to perceive two completely separate things at the same time.
Either that, or just give them all control over the recipient's dreamscape — fighting for control. I can definitely see a group of Wizards arguing as they try to build the dreamscape around them; or more appropriately, using the target's dreamscape to set up a bar, having decided it's a fabulous meeting place since the psychological decorations are top notch and the existential angst is tuned to just the right frequency.
Best Answer
Both Mind Blank and the Ring of Mind Shielding provide an absolute protection against the Dream spell.
Mind Blank's text (emphasis mine) states:
This last line provides an absolute protection against any spell that would affect the target's mind.
Dream states (emphasis mine):
The very first sentence of Dream means the entire spell falls under the catch-all of "spells or effects of similar power used to affect the target's mind or to gain information about the target" at the end of Mind Blank.
If that wasn't enough, both:
would fall under the definition of affecting the targets thoughts, and thus be prevented by Mind Blank.
The Ring of Mind Shielding is more complicated, since it's not quite as explicit. It's text states:
Now we ask two questions:
Certainly, being able to enter and observe your dreams would count as reading your thoughts (as the messenger will remember the dream they observed).
While Dream doesn't explicitly say the communication is telepathic, the communication initiated by Dream is mind to mind communication. This is pretty much the definition of telepathic communication, and as such would be blocked by the Ring of Mind Shielding.
Is there anything else that provides such a protection?
In the Waterdeep: Dragon Heist adventure sourcebooks there is a magic item that provides this protection:
The item's description states, in part (p. 191):
In addition, an Antimagic field will protect you absolutely from the Dream spell (emphasis mine):
Since Dream is a spell it cannot protrude into an Antimagic Field.
Finally, any spell or effect which allows you to temporarily be on a different plane of existance, like Etheralness, Mordenkainen's Magnificent Mansion, or Demiplane will protect you as they prevent the requirement of the Dream spell that: