The quest begins in Cianwood City where you will have to talk to Eusine, he can be found directly North of the Pokemon Centre.

Eusine will challenge you to a battle which you must win to begin the quest, his team will consist of Drowzee, Haunter and Electrode.

When defeated you will then be able to find Eusine on Route 42 between the two bodies of water and slightly south of Hiker Benjamin(shown below)

You will need to talk to Eusine again which will lead to an interaction with Suicune, you will then get a new side quest; Legendary Suicune which will direct you to the Dragon’s Shrine in Blackthorn City.

The quest continues upon entering the Dragon Shrine(this will require the HM Surf and Whirlpool), your quest will update to say “Read the lines written on the pillars (0/4)”. This can be completed by interacting with the first four pillars inside the Dragon Shrine while standing in the spots marked in the below image.

This will cause your quest to update again, directing you to “Meet Eusine in Mt.Mortar”. The easiest route to find Eusine in Mt.Mortar is by using the entrance found on Route 42 North of where you talked to him earlier.

To reach Eusine inside Mt. Mortar follow the map below(red line marks the easiest path). You will need HM Surf and Waterfall and to have obtained the 8th Gym Badge in the Johto region.

When you reach Eusine he will challenge you to another battle his team consists of Hypno, Gengar, Electrode, Gengar, Hypno between levels 45-50.

Once defeated Eusine concedes that you should be the Trainer to catch Suicune, this will cause Suicune to spawn nearby, your quest will also update to “Challenge Suicune” which you can now do by simply interacting with him.

Challenging Suicune will complete the quest, you can choose either to catch or defeat him, if defeated you will need to wait 1 week to try again.