Very Rare Item: have I got the power level right?

[KorvinStarmast]

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

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. Granted, raritity gives us a range, or zone, of power in 5e, but it's still one way to guess about right.

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 its wearer:

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

Additionally, the mantle has seven charges which refresh each day at sundown (or at dusk?). 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, at the wearer's location, 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?

Spoiler: Artistic and Thematic Notes

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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. The FR god Helm is called The Watcher, and this item is not intended to be one of his creations).

@PhoenixPhyre:

Spoiler: campaign context

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this is an item that was inspired by Rune Al'Thor and his relationship with The Watcher as a Gleaner.

What say ye, Playgrounders? Have I got this about right? Are there some tweaks you'd recommend?