The Watcher's Mantle: is very rare the correct rarity for this item?

Question

This item is being designed for a campaign where The Watcher is the patron of Hexblade warlocks. He is roughly a demigod and fulfills the role of The Grim Reaper. I want to make it portable to anyone's campaign by getting the rarity and powers as close to right as I can. I am using as bookends the Staff of Power (very rare, high end of the power level) and the Robe of Stars (a lower power level very rare item) to try and find the right power zone for this mantle.

Watcher's Mantle
Wondrous Item, very rare, requires attunement
The mantle is made of deep black woven wool trimmed with black silk. When attuned, it grants the following benefits to it s wearer:

• A +1 bonus to all saving throws.
• Dexterity (Stealth) checks are made with advantage.

Additionally, the mantle has seven charges which refresh each day at sundown. Spells using the mantle have a spell save DC of 17. When attuned, the wearer can cast the following spells (charges in parentheses): cause fear (1), misty step (2), darkness (2), enervation (5). When the wearer is targeted by a melee or ranged attack, the mantle can be used to cast darkness as a Reaction if there are enough charges remaining.

I am considering adding one more feature, but am not sure if this would make the cloak too powerful.

Cause fear can be cast as high as level 5, using one charge for each spell level, if there are enough charges remaining.

I am also not sure if the spell save ought to be DC 16 or DC 17.

Bottom line: is adding the additional feature a case of adding too much power for a very rare item?

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Artistic and Thematic notes:

• The deep black color is based on the cloak worn by the protagonist in Gene Wolfe's The Shadow of the Torturer/Book of the New Sun, but any 'midnight black' color is thematically appropriate.
• This could also be called Shadow's Mantle, since The Watcher in our game is the Lord of the Plane of Shadow. In any campaign it could be linked to Shadowfell, a death deity, or other similar power / spirit / demigod. (@ThomasMarkov pointed out that the FR god Helm is called The Watcher, and this item is not intended to be one of his creations).

Answer 1

I could find only 1 cloak in the printed materials that was very rare - The Cloak of Arachnida (DMG 158). There are properties of your item that exist on cloaks already in print - the stealth advantage is on the rare Cloak of the Bat (DMG 159), and the uncommon Cloak of Protection (DMG 159) gives the same saving throw boost. Looking at the utility of these existing items and their rarity I believe that very rare is the right place to put your item. I don't think that the additional feature would be that significant a change in the power level, trading charges for levels in that way seems decently balanced in the case of cause fear.

The only issue I see with your item is the Spell Save DC on spells cast with it, 17DC is very high. Spell saves max out without boosting from feats and items at 19 (8 base + 6 proficiency at level 20 + 5 stat mod at 20 cap), so 17 is nearly maxed out as it is. Note that the Web spell cast by the existing very rare Cloak of Arachnidia has a spell save DC of 13. 16 may also be higher than you should use in that case.