Keys can refer to several different types of items in World of Warcraft: Classic. Most, with the exception of the "quest item" type, will fit on the player's keyring.

Regular keys

These can be used to unlock a specific or general type of locked container. Blacksmiths can create skeleton keys and use them (skeleton keys require the same blacksmithing skill to use as to create). These are destroyed upon use.

Instance keys

These are usually soulbound (often a quest reward) and can be used to unlock entryways. These keys are not destroyed upon use. Unlike attunements, players do not need these keys to actually enter the instance, only to make use of shortcuts or unlock special events.

The Scarlet Key

Opens the Armory and Cathedral sections of the Scarlet Monastery. Also used to open the door in the Scarlet side of Stratholme. It is found in Doan's Strongbox behind Arcanist Doan in the Library section of the Monastery.

Workshop Key

Opens the backdoor to Gnomeregan, which lets players skip most trash mobs. It is found on the corpse of Electrocutioner 6000 in Gnomeregan.

Prison Cell Key

Opens prison cell doors in Blackrock Depths. Needed for Alliance and Horde quests like Commander Gor'shak] and Marshal Windsor. It drops from High Interrogator Gerstahn], who can be found in the prison section. Be sure to kill patrols and mobs in the tunnel before attacking Gerstahn, as she uses fear attacks. Also, the key doesn't pop up for rolling on, so agree who takes it in advance.

Shadowforge Key

Opens shadowforge doors in Blackrock Depths. Allows players to skip things like the Ring of Law and gain access to the bar.

When entering Blackrock Mountain, players need to walk across the chains to get onto the center island. Inside the building is a tomb, on top of which stands Franclorn Forgewright who gives you the quest Dark Iron Legacy.

Players need to be dead and a ghost themselves to talk to him, however. The quest requires them to kill Fineous Darkvire and retrieve Ironfel. Once accomplished, find the statue of Franclorn Forgewright and use the hammer on it.

Crescent Key

Opens the doors to Dire Maul North and West. Enter Dire Maul East from the center courtyard. There is an imp, Pusillin, who will repeatedly run when talked to. When he's finally cornered players can fight him for the key.

Skeleton Key

Opens the door to Scholomance. Players need to complete the quest Scholomance, obtained from High Executor Derrington (Horde) or Commander Ashlam Valorfist (Alliance).

A good place to gather the 15 skeletal fragments is Felstone Field where reasonably low level skeleton casters can be found. Eventually players will end up in Tanaris where they need 15 gold and 2 thorium bars as part of the quest Fire Plume Forged.

After completing the quest, return to the original Scholomance key quest giver. The last part is retrieving Araj's Scarab] for the head of the key.

Key to the city

Allows opening of the service entrance gate to Stratholme. Players need to kill Magistrate Barthilas] in Stratholme on the undead side.

Quest item keys

One example of a quest item key would be the Defias Gunpowder required for the cannon near the end of Deadmines. It is picked up and consumed each dungeon run. It is not exactly a quest item, but it is the only way to open the cannon, it is consumed there, and it cannot be used anywhere else.

Mallet of Zul'Farrak

When going to the instance Zul'Farrak in Tanaris, players can pick up a quest beforehand in the Shimmering Flats called Gahz'rilla. However, to summon the boss and complete the quest, they will need the Mallet of Zul'Farrak.

To obtain it, players must travel to the Altar of Zul in the Hinterlands. On top of the temple are two elites that must be killed before getting the Sacred Mallet from the corpse of Qiaga the Keeper. Players then travel to Jinta'Alor, also in the Hinterlands. On the top of this temple is another altar. Stand near the altar, and click the Sacred Mallet to convert it into the Mallet of Zul'Farrak.

Yeh'kinya's Scroll

Used to summoning Hakkar in the Sunken Temple. This is a way to resummon Hakkar. Obtaining the Scroll starts with the quest Screecher Spirits given by Yeh'kinya at Steamweedle Port in Tanaris.

Scepter of Celebras

This allows players quick access to Princess Theradras in Mauradon. Before entering the Mauradon instance, they will have a choice of entries. They can go into the orange or purple areas, or go straight ahead to the first Kahn. Behind the first Kahn is a small altar. Use the Scepter on the altar create a teleporter. Only one person in the group needs it.

Obtaining the Scepter begins with the quest Legends of Mauradon, given by Cavindra], a female satyr standing in a pool outside the instance in the orange section of Mauradon. Kill Noxxion and Lord Vyletongue to get both parts of the scepter. Then find Celebras the Cursed and kill him; he will then transform into Celebras the Redeemed]. Keep talking to him to complete the scepter.