The Skeleton Key. This artifact of Nocturnal is said to imbue the user with abilities, not only to unlock every physical lock, but also unlocks unknown abilities within the wielder.

But then you get the key and it turns out it's just an unbreakable lock pick. It can't even unlock doors that require keys.
I don't know about you, but I was very disappointed upon finding this out.
All that work to get it.
All the talk about how important it was. 

So, this will reward the player with some new abilities and a powerful and versatile spell.

Once you pick up the skeleton key from Mercer Frey's soon-to-be-waterlogged corpse, you will get a spell book in your inventory.
Once you read that book and learn the spell, you will be given The Skeleton Key's Gift book which will detail these abilities in-game.

Passive abilities:
Learn effects of ingredient object when picked up.
Become invisible when sneaking and combat conditions change.

Claim spell affects current object highlighted by your crosshair.
 * Become object's owner
- Everything below is optional and configured in the settings file -
 * Learn ingredient effects from flora, tree and ingredient objects without picking them up.
 * Earn small amounts of alchemy exp. 
 * Learn enchantments (temporarily) from armor and weapons.
 * Earn small amounts of enchanting exp.
 * Recharges weapon enchantments (effect is temporary for base items, permanent on player-enchanted items)
 * Increases weapon charges (effect is temporary for base items, permanent on player-enchanted items)
 * Increases weapon speed (effect is temporary for base items, permanent on player-enchanted items)
 * Owns and renames horses 
 * Unlocks locks/doors/traps
 * If a lock requires a key, provides that key.
 * Earn lockpicking exp
 * A few various actions when used on an npc:
 * - If a child, push them with enough force to launch them in the air (force is set in settings)
 * - If horse, own it & rename it
 * - If other npc, kock down with minimal force
 * - Optional sneak force bonus.
 * - Optionally launch them just like children

Settings file: [Default settings]
{
"learn-ingredients": false,
"earn-alchemy-exp": false,
"learn-enchantments": false,
"earn-enchanting-exp": false,
"pick-locks": false,
"get-keys": false,
"earn-lockpick-exp": false,
"own-horses": false,
"rename-horses": false,
"recharge-weapons": false,
"set-weapon-charges": 100,
"boost-weapon-speed": false,
"boosted-weapon-speed": 1,
"knock-npc-down": false,
"launch-children": false,
"children-force": 10,
"launch-adults-too": false,
"clear-cell-owner": false,
"player-is-cell-owner": false,
"invis-sneaking": false,
"hotkey-g": false,
"give-spellbook": false,
"sneak-force": 1
}
To enable any feature/setting, replace false with true.
"children-force" will have no effect if "launch-children" is set to false.
"launch-adults-too" setting uses "children-force" amount of force. Regular force applied to NPCs is 0.5 for reference. This just ragdolls them and allows you to walk through them for a second. Useful for NPCs blocking doorways. Increased amounts of force tend to push characters in random directions very violently, mostly into the air.
"give-spellbook" if set to true will give the player the Claim spell book the first time this mod is enabled. It will set the global variable and allow the player to have all the abilities without needing to go through the Thieves Guild quest line.
"hotkey-g" if set to true will allow the Claim spell effect to be hotkeyed to the G key. This option also bypasses the need to obtain the actual skeleton key through the quests.