The Tiny Cloak of Darkest Night provides a 1.5s cast invis, compared to the normal 5.5s cast on standard invis items. It drops from the Caller Event, making it one of the most valuable quality-of-life items a bard can farm for themselves.
Video guide: Caller Event / Tiny Cloak of Darkest Night — credit to Lethan the Bard Guy.
The event involves multiple caller mobs and the boss Katcha. The general approach is to whittle down the group efficiently before isolating Katcha for a clean kill.
When Katcha spawns at the front, pull him directly and use your full toolkit: charm, mez, mana drain, and largos. This is the cleanest version of the fight.
Suicide charm the casters into the group of mobs to burn them down. Once it's just Katcha and non-casters remaining, do a regular reverse charm and dirge him down at 2% like normal.
You can potentially swarm the group at the mouth of the cave entrance, but use caution: Katcha casts and can kill you if he still has mana. A single hit from any mob could snare you, which is likely a death sentence.
Pulling Katcha is stressful every time, but you have plenty of tools available: charm, mez, mana drain, and largos. Use them liberally to stay in control.
Eventually you'll be 1:1 with Katcha. At that point it's just killing a strong mob. Run a slow circle and swarm him down once he's out of mana.
If you don't go far up the cave entrance you won't despawn the callers, keeping the event active. If the cloak doesn't drop, rinse and repeat — keep the event going until it does.