Walkthrough

1. Accept the quest from your trainer in Job Center and get a Training Badge.

2. Meet Officer Bao to know what happened around Ape City.

3. Kill the Macaques outside Ape City and you will have a chance to obtain a Decayed Heart. Bring it to Officer Bao but he cannot do anything to it. He told you to ask Royal Doctor Li for help.

4. Royal Doctor Li extracts a bottle of Poison out of the Decayed Heart and suggests you transform into a Monkey to test the Poison. After being transformed a monkey, you can right click the Poison to use. Suddenly, you are teleported to somewhere.

Note: You should wait there 60 seconds to return to human being again. And then kill some monsters to be sent back to Royal Doctor Li automatically.

5. You come around seconds later. Royal Doctor Li asks you to bring the Poison and kill the Venomous Apes at Ape City (800, 470). You will obtain a Devilish Sword after killing the Ape Leader.
Note: When you summon the Venomous Apes, you have to kill it by yourself. If it is killed by others, you have to ask for the Poison from Royal Doctor Li again. If it is killed by others for more than once, you have to start over this quest from your trainer. 

6. Bring the Devilish Sword to Officer Bao and get an Ultra Badge so you can swap it for a reward from your trainer in Job Center.