You can begin this Hunt after completing the Lhusu Mines and after Vaan has complete the Notoriety mini-quest posing as Basch. The Mark, Nidhogg, is back in the Lhusu Mines.
Aekom who can be found standing just outside of the Lhusu Mines near the Save Crystal.
Nidhogg is located inside the Lhusu Mines in Transitway 1. It can be seen almost immediately upon entering the area as it sits in at the split pathway between the two large roads heading west. There is a Steeling flying around nearby but it is relatively easy to take out the Steeling while fighting Nidhogg.
Nidhogg is one of the easier marks to defeat. Its attacks become much stronger though once it drops below 80% HP as it gains access to a new attack called Tail Spear. It will also be able to cast a buff called Saber which will increase the damage that it does while also increasing the damage done to it.
This mark is susceptible to all kinds of negative status effects including Blind, Slow, Immobilize and Silence. At a minimum, make sure to setup a Gambit to have one of your characters keep Blind on Nidhogg to help you deal with any unexpected bursts of damage. You should also setup a Gambit to deal with the Poison status effect (such as Ally: Any > Poisona or Ally: Any > Antidote).
Fire Magick is very effective against Nidhogg if you are having difficulties taking it down.
A video of the battle in the Zodiac Age version is included below:
Aekom will also give you a ‘memento’ called a Great Serpentskin. Be extra careful with the Great Serpentskin! It is used in the Desert Patient side quest to obtain items and it can accidentally be sold at shops (which happens to many players).