To summon the Raven Lord Anzu, you had to fulfil two requirements.
The first was an essential item – an Essence-Infused Moonstone. This was a quest item that could be obtained from a Druid flying quest. It wasn't that hard to activate this quest. There were quite a few Druids who had reached this stage.
After this, this Druid would have to learn Strengthened Stealth.
This wasn't actually too difficult. Strengthening Skill Books were highly sought after, but with so many players in Dawn, quite a few people were able to obtain them through a variety of methods.
Both Hachi Chan and Innocent Child of Ruling Sword had the Strengthened Stealth skill.