Black Cleft

The usefulness of this charm in actual gameplay is next to none:

• Makes you more vulnerable to magic attacks by a non-insignificant percentage.
• Takes up 3 inventory slots.
• Removes the immunity on a grand total of 6 monsters in the whole game, one of which is uber-tristram only.
• Makes you deal 5% of your total damage to those enemies. This number is currently impossible to increase.

List of magic immune monsters affected by

Black Cleft. Only notable magic immune enemy is

Achmel the Cursed and his minions in second

Baal wave in

Throne of Destruction:

It should be noted that

Holy Bolt and

Fist of the Heavens, which can only deal their magic damage to undead or demon targets, ignore the magic resistances or immunities of those targets. Because five of the six mob types that are immune to magic are either demons or undead, all three super unique monsters that are immune to magic are undead, and no combination of base monster and affixes can result in a unique monster that is immune to magic, this near-useless item is not even beneficial in what would otherwise be one of its few use cases.

The only means of partially counteracting this item's magic resist penalty is an item created using

Recipe: Safety Shield. Such shields provide +5-10% magic resist, well short of the substantial penalty

Black Cleft imposes.

This is the exhaustive list of skills that technically benefit from this:

Magic Arrow,

Psychic Hammer,

Berserk,

Teeth,

Bone Spear,

Bone Spirit,

Holy Bolt,

Blessed Hammer,

Sanctuary Aura,

Fist of the Heavens. While at face value this might make this item seem valuable, in truth nearly any build that would be using these skills for damage will already have a practical backup:

• Magic Arrow &
  Berserk scale off of existing physical damage, so you can simply use a physical attack. (which can benefit from
  Amplify Damage)
• Psychic Hammer is useless as a damage skill by Hell difficulty.
• Bone Necros (or all necros, really) have access to
  Corpse Explosion, and in all situations there will be non-immunes you can kill and explode to get the immunes.
• Sanctuary,
  Holy Bolt, and the
  Bolts from
  Fist of the Heavens ignore the magic immunities of undead inherently. The only non-undead magic immune is the Act 3 Wailing Beast, which is an animal-type and not affected by these skills regardless.
• Because of the immunity-ignoring effect of
  Holy Bolt, this likewise means that Wailing Beast is the only enemy of note a Hammerdin can't damage.

In practice, hammerdins are basically the only build that has a real worry about magic immunes... And it's only for a single monster found in at most three side zones, with only one being needed for a quest:

Ruined Temple. This isn't really a farmable zone, so this is effectively a one-time run to grab Lam Essen's Tome. Ultimately it's easier to just try and run past the enemies, or enlist multiplayer help. This is the closest to useful this charm gets.