This
missable achievement requires you to gift a specific flower to Shadowheart. It may seem like a simple task, but it requires some time investment through the main story and certain decisions must be made. The first opportunity to find a Night Orchid will be in Act 2, but it’s possible further into Act 3.
Below you'll find a guide about your companion: why this is missable, how you can increase your likeness with her, flower locations and other stuff. There will be spoilers regarding Shadowheart's arc to explain certain situations. Please bear with me!
1. Requirements:
*= See more optional info and plot development clicking the spoilers below.
- Play on a file where your main character isn’t Shadowheart.
- You must save Shadowheart the first chance you got (which is "Prologue: Escape the Nautiloid") and recruit her in your squad. There will be several encounters if you miss or ignore her after this, but ultimately, it will shift her attitude against you.*
- During “Gauntlet of Shar” dungeon in Act 2, don’t let Balthazar take Nightsong.****** Spoiler - click to reveal ***If you don’t rescue her on the Nautiloid, she can still be recruited by finding her near the Overgrown Ruins Waypoint, just north of the beach. By missing both chances, you can find Shadowheart again in the Druid Grove, sitting near the oxen. She'll be here regardless of if you saved her from the Nautiloid or not, it's just her reaction to you will be slightly different.
Assuming that you've decided to not join forces with Shadowheart yet, she will once again find you as you're leaving towards the Mountain Path, with the artifact in tow. She will wait in this position until you change your mind and recruit her.
And then, even after all of this, if you decide to take the artefact by force, Shadowheart will become hostile and you will be forced to kill her. Interestingly enough, she will be susceptible to revive, but she'll turn hostile immediately afterwards, meaning you've locked out your chance to recruiting her.
It's unclear whether she can catch up to you if you choose to proceed through the Underdark or before entering the Shadow Cursed Lands. But at this point, I'd suppose either you don't want the achievement or you're not recruiting her for fun you soulless heart! 
***** Spoiler - click to reveal ***This only applies assuming you haven't given the flower to her yet. The achievement can be unlocked before this encounter in Act 2. Let's say you still need to work on her likeness / approval towards you, or you forgot to search the flower by whatever reason, then you must ensure Shadowheart remains with you for Act 3 so you can try without issues.
Shadowheart will leave your party permanently if you let Balthazar take Nightsong. There are two paths you can take to progress: either convince Shadowheart to spare Nightsong (don't fight her, she'll get mad at you which will result in a battle and thus ruining your friendship) or let her kill Nightsong fulfilling what Lady Shar asked of her.
By sparing Nightsong, Shadowheart will later turn into a Selunite for Act 3. But if she killed Nightsong, Shadowheart will turn into a committed Shar follower. Romance is still possible in either scenario, but the "good path" so to speak, is by saving Nightsong.
2. Getting to know Shadowheart:With Shadowheart in your party, you’ll need to work on getting your approval up with her. She generally likes being a kind person, some examples are: if you choose to help others, use deception / persuasion to avoid combat and show kindness to animals. You can also gain free approval from asking about her (just don’t pry when she gets defensive) respecting her boundaries.
Once her approval is relatively high, she will come forward and tell you she is a Shar worshipper. React calmly and tell her you don’t care who she worships.
From here, you’ll be able to ask about her again and this time an option should come up saying you want to know about her outside the parasites and her Shar worship. If done properly, she’ll tell you that she loves night orchids and can’t swim.
Now that you know this, you’ll have the ability to gift her a Night Orchid.
3. The search for the Night Orchid:Note that you will only need one Night Orchid to complete the quest. Act 2 has fixed locations where the flower appears, whereas Act 3 have a bit of random places which vary from player to player.
Act 2 Locations:*** Spoiler - click to reveal ***
- Ruined Battlefield:
Relatively close to the entrance to the Shadow-Cursed Lands from the Underdark. More specifically, North of the Shadowed Battlefield waypoint (X: -56, Y: -175). If you’re having trouble finding it may be because it blends so well with the tree, so you may want to highlight any valuables in the area. If it’s not there, take a long rest at your camp and come back later.
- Shadowed Battlefield:
Just a few paces North-West of the Shadowed Battlefield waypoint. There, you will find a tree with twisted branches, jump onto the tree, and highlight the zone. (X: -36, Y: -40)
- Near "He Who Was":
There is a body of Harper near the camp where you first met "He Who Was", during "Punish the Wicked" side quest. Interact with the body and loot it to obtain the third and final Night Orchid during Act 2. (X: -93, Y: -100)
Act 3 Locations: *** Spoiler - click to reveal ***
- Cazador's Dungeon:
During Astarion’s companion quest, "The Pale Elf", make your way to the bottom level of Cazador’s dungeon and interact with the trunk at coordinates (X: -1950, Y: -860).
- Cloister of Sombre Embrace (House of Grief):
https://imgur.com/a/77O8C6Y
In the southeastern room of the Shar Cloister's central chamber. You should notice a wooden barricade on the wall. Use your weapon to break it open and discover a ladder, climb it and go inside. (X-370, Y:-1685)
- Near the City Sewers:
https://imgur.com/a/sTRUZc4
Head north and you will find a bunch of grease enemies. After completing the fight, head up to the grated iron doors and you will have to pick the lock. The chest behind may have a Night Orchid, but this is one of the random locations.
These are all the locations I could find. It should be enough, but feel free to share more if you want once you have discovered them.
4. Giving the flower to Shadowheart:Once you pick up a Night Orchid, talk to Shadowheart and there will be a special dialogue option. Choose to bring up Night Orchids and then gift the one you found to her. Doing so, you will gain her approval and the achievement. She’s a softie after all, huh?
TLDR:- Don’t choose Shadowheart as your main character at the start.
- Save her as soon as you can (Prologue) and recruit her in your party.
- Gain her approval by being a kind person throughout the story.
- Be supportive when she opens up to you about her past.
- Decide the fate of Nightsong but don't let her be kidnapped by Balthazar.*
- Find a Night Orchid in any of the suggested locations.
- Gift her the flower you’ve collected.
*Only applies if the achievement has not been unlocked before the "Gauntlet of Shar" dungeon in Act 2. Or if you want to give the flower to Shadowheart later in Act 3 by whatever reason.
If this guide was useful in any way, please leave a thumbs up, I'll appreciate it a lot!
