There is a little side story for Hendrik back at Heliodor Castle, so head there and go inside. Before doing this little side story, you will want to get Hendrik resistant to the dark element. You should have more than enough equipment during the final part of the game in order to get it high enough, or even 100% resistant to dark. This is because there is a one-on-one battle at the end and being resistant to dark will make it a lot easier. The Field Marshal’s Finery offers 40% protection at +3, and the Ruinous Shield offers 50% protection at +3. Once you’re ready, enter Heliodor Castle and speak to the guard in the main hall with a pink speech bubble, who will mention about a shadowy thing floating about the place.
The last place it was seen was the courtyard, which is off to the left of the main hall, and where the Yggdrasil root is located. Go there now and you will see the shadowy figure, with you being told to touch the root. Do that to get a vision, which should pretty much tell you who the shadowy cloud is. The shadow will now move, so follow it back into the main room, before it ventures north. It will head east at the first intersection, then enter the dining hall. The shadow will continue into the small room on the left, so interact with it in front of the one shelf. The shadow will disappear now and Hendrik mentions going to Jasper’s old room.
This is off the left side of the main hall, to the north of the courtyard; just look for the pink marker on your map. Go into Jasper’s room and interact with the shadow, only for it to disappear once more. Read the journal on the table after it’s gone to learn more about Jasper’s true feelings from the past. Once you’re done there, head back into the main hall to find the shadow, who will disappear when interacting with it. The shadow will move to right outside the throne room, then head upstairs. Follow it and it will eventually go outside to the balcony, which is where a cutscene will take place, as well as the battle.
BOSS - Shadow Jasper
The fight is one-on-one between Hendrik and Jasper, so depending on when you do this, it could be an easy battle or a harder one. Despite that, equipping items that protect Hendrik from dark will help immensely in mitigating damage, making things a lot easier. The fight is in three phases, with Jasper starting out in human form, with twin swords. He can attack normally, with it hitting twice, and will also slam a dark sword on you. Depending on how well built Hendrik is, you may be taking little to no damage, since he does have a lot of defense. If you feel like you need to heal at all, Hendrik does have access to Moreheal.
After taking enough HP off of Jasper’s human form, he will transform into the Jasper Unbound form, which is the one with wings. Sometimes he will waste a turn “guffawing graciously,” which is always nice. Besides his normal attack, he can toss a lance of darkness at you, which deals no damage if you’re fully protected from dark. There is also a dark-breath attack, as well as a flurry of slashes. Note that Jasper can also remove your buffs with the disruptive wave here, so unless you need the buffs to survive, it’s better to ignore them. Once enough damage has been done, Jasper reverts to his human form and the fight will be drawing to a close soon.
The third phase is the easiest, as Jasper is beginning to deteriorate here. The darkness consuming him will begin to damage him on its own, and the majority of his attacks will likely miss or not do much damage. Continue hitting him and the fight will end. Throughout the battle, depending on how you built Hendrik, you should be using your best single-target attacks. If using an axe, then Parallax is definitely the best option. Unbridled Blade is great for greatswords and for swords, there is Gyrfalcon Slash.
When you’ve won the battle against Jasper here, watch the scene that follows. This will bring a close to things and your reward will be the Twin Eagle Armour. While the armor itself is not the best, it is also a costume for Hendrik and if you plan on getting the platinum/all achievements for the Definitive Edition, you will need it.