I’m working on a magic staff based on lore of the Hawthorn Tree. The staff has 3 properties which the users controls through the staff’s thorny hand. Effectively, the user can use charges to:

1. Grapple and deal bonus damage on subsequent rounds (1 charge)


2. Deliver necrotic damage and regain Hit Points, once per day (3 charges)


3. Use the staff as the Mage Hand cantrip (0 charges)

Blackthorn

Requires attunement by a cleric, druid, or warlock

This staff can be wielded as a magic quarterstaff that grants a +2
bonus to attack and damage rolls made with it. While holding it, you
have a +2 bonus to spell attack rolls. This staff deals piercing
damage rather than bludgeoning damage, but otherwise retains the normal
properties of a quarterstaff.

The staff has 10 charges that can be used for the following abilities.
Once per turn, the user can take 1 of the following actions while
holding the staff:

• Thorny Grasp. When you hit with a melee attack using the staff, you
  can expend 1 charge to grapple a target. On a successful hit, the
  tangle of thorny branches at the end of the staff forms a large hand
  and grasps the target. If the target is large or smaller, it is
  grapped (escape DC 17) and restrained until the grapple ends. On
  subsequent turns, while restraining the target with at least one hand
  holding the staff, you can use a bonus action to deal 1d6 piercing
  damage. Until this restraint ends, the user can't use the staff on
  another target.




• Death’s Touch. When you hit a creature with a melee attack using the
  staff, you can expend 3 charges to force the target to
  make a DC 17 Constitution saving throw. On a failure, the target takes
  an extra 4d6 necrotic damage, and you regain a number of Hit Points
  equal to half that necrotic damage. This property can't be used again
  until the next dawn.




• Helpful Hand. You can use your action to control the hand and perform
  simple tasks with a reach of 10 feet. The hand can't wield a weapon,
  activate magical items, or carry more than 30 pounds. This maneuver
  does not use any charges.

The staff regains 1d6 + 4 expended charges daily at dawn. If you
expend the last charge, roll a d20. On a 1, the staff becomes a
nonmagical quarterstaff.

Based on items in the DMG, this staff seems to fall somewhere between a Staff of Withering (rare) and Staff of Power (Very Rare). Is my estimation correct? Would lowering the DC to 15 from 17 also lower the rarity?

Very Rare

There are a lot of similarities of this staff to staves like the Staff of Fire (Very Rare) and that of the Staff of Power (Very Rare).

The Staff of Power provides the +2 to attack/damage/Spell rolls that yours does as well as additional casting capabilities that are quite powerful. The Staff of Fire doesn't have the +2, but it does have a similar option of delivering additional damage on a strike.

The next level up would be a Legendary staff like the Staff of the Magi. The biggest uptick in power here is in the number of charges (50) and breadth of available casting options. Yours does not have this.

Comparing the Very Rare to Legendary options, yours fits quite nicely in Very Rare.

Lowering the DC

I'm not sure if lowering the DC would be the impactful, but you may want to consider dropping the built-in DC and use the spellcaster's DC instead like the standard magical staves of this level do.