Nethaera's Light
Source: Drop. The lost magical light of a Dalaran mage, or sentient candle with a positive attitude? You decide. Magic type. PLEASE NOTE: This NPC is in Dalaran-NORTHREND, not the revamped Broken Isles version of the city.
Nethaera's Light is a non-combat pet found in Dalaran (Northrend) that you obtain by targeting a wandering candle NPC and repeatedly using /cheer until it adds itself to your Pet Journal.
Guide awaiting review
Travel to Northrend Dalaran
Go to Dalaran in Northrend — not the Broken Isles version of the city. These are two separate versions of Dalaran, so confirm you are in the Northrend one before searching.
Locate the candle NPC
Use /tar Nethae to target the Nethaera's Light candle NPC as you move around Dalaran. Dismount before attempting to interact, as you must be on foot and in range for /cheer to register.
Spam /cheer at the candle
Once the candle is targeted, repeatedly use /cheer — the number of cheers required varies. A confirmation message will appear in your chat log when the pet has been successfully added to your Pet Journal.
Watch for candle despawn
The candle despawns after being cheered at, and respawns roughly every 5 to 10 minutes. Multiple characters can receive credit from the same candle spawn.
Tips
Watch for other players' Nethaera's Light battle pets nearby — those will appear with a pet label and cannot grant you the drop, only the actual candle NPC can.
You can use a two-macro method: one macro to clear your target, target the candle with /tar Nethae, and mark it with a raid icon, and a second macro to spam /cheer while watching for the candle's brightest flash as your cue to click.