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.
Jump does not let you bypass the 'Speed' restraint on jumping. But the ring is still useful
As to Jump (and the boots) not increasing your speed, here is some Developer commentary on the matter, specifically addressing these effects.
Question posed: "Can you jump farther than your movement when using magic i.e spell Jump & boots of striding and springing?"
To be clear, things like the jump spell don't increase speed. You can jump crazy far, but your speed caps it. -Crawford
Are you saying you can't jump farther than your speed even with Jump spell or Boots of Striding and Springing? -Followup Question Asked
Every foot jumped costs movement, so you can jump farther than your current speed if you take the Dash action. -Crawford Again.
Source
So, no. Neither the Jump spell nor the Boots of Striding and Springing allow you to exceed your total Speed in distance jumped, though taking the Dash action allows you to move your full Speed a second time during the turn, effectively increasing your Speed by 2x for the round. This lets you actually get some use out of your massively boosted jumping distance. This would rather plainly imply that the Boots of Striding and Springing are generally superior to a Ring of Jumping.
Where this is not true is when you start combining magic items. For example, if you get your hands on a pair of Boots of Speed. Naturally, you can't wear two pairs of boots...but you can wear a pair of boots and a ring. This combination takes two bonus actions to fire up, but then you have a full minute of doubled running speed and tripled jumping distance, without having to take the Dash Action. And because you can get 10 minutes of boosted speed from the boots per day, you can do this 10 times a day. So you have all the mobility of someone Dashing around in Boots of Striding and Springing, but still have your Action free to use.
So, taken in isolation, it does appear that the Boots of Striding and Springing are generally superior to a Ring of Jumping. However, you have 10 fingers for ring-wearing, but can only wear one pair of boots. So the ring is better for combination work.
Best Answer
Insight checks are non-magical.
The description only states that you're immune to magic that does those things. However, an insight check is not magical (PHB 178):
Thus, the ring does not protect you from ordinary, non-magical insight checks.
As for DCs, there aren't any specific rules as to what the DC should be. Given that the interaction is wholly nonmagical, I usually use a contested deception vs. insight check when a scenario like this arises.