Currently, there are mixed reports. There are a few Reddit threads tracking anecdotal reports of when it does/doesn't trigger--here's one I've been using--which mostly agree on major points and bring up a few that may or may not count.
Since some people report the achievement not triggering when tieflings not on the list die, there is a nonzero chance of a bug having been present at some point; there are also a few moments of chaos where it's easy to miss a single refugee fatality.
The tiefling refugees pop up in all three acts, and most of them also have either a quest or a conversation in each act that requires player follow-up. The actual saving generally occurs in Acts 1 and 2, but it looks like you have to tie up all loose ends by finding and speaking with each group of the survivors in Act 3 to properly trigger the achievement.
The following tieflings are generally accepted as required, and confirmed in my own achievement-triggering playthrough:
- Overall points of potential failure/events impacting a whole group:
In Act 1, the druid grove should not be raided by the goblins, the Rite of Thorns cannot be completed, and any tieflings who join the initial combat upon discovering the grove should survive.
In Act 2, no refugee tieflings can die in the raid on Last Light. No tiefling prisoners can die in the escape from Moonrise Towers prison level--some reports claim that the prison break must occur the same 'day' you discover it (before you long rest), others simply say it must occur before the Shadowfell cutoff of all unfinished business.
The general group of tiefling survivors can be found in the refugee area of Rivington in Act 3 for final conversations.
The tiefling by the makeshift prison threatening a caged goblin with a crossbow. She needs to be talked out of killing the goblin in revenge for her brother's death; if she is not, she seeks further vengeance outside the Grove and ends up dead. She does not reappear in later Acts.
Act 1, she needs to be freed from custody in the druid grove so Kagha does not execute her.
Act 2, you can find her in Reithwin in the Shadow-Cursed Lands, looking for her parents, who have not survived. Tell her what happened and send her to your camp. She will be upset, and after discussing with Withers, will depart in a calmer state of mind.
Act 3, you can find her in the sewers for closure conversation.
- Rolan, Cal, and Lia (All Acts)
Act 1, mediate the argument between the siblings and convince them to stay with the refugee group instead of heading off early.
Act 2, you have to speak with Rolan in Last Light to get the quest to find his kidnapped siblings in the Moonrise Towers jail. Then, you have to find and save Rolan in the Shadow-Cursed Lands in a proximity-based encounter with shadows.
Act 3, Rolan is in Sorcerous Sundries. Depending on your choices in the Nightsong quest in this Act, he may join in a combat against Lorroakan.
- Lakrissa and Alfira (All Acts)
Act 1, if the PC is Dark Urge, Alfira must survive a scripted camp encounter.
Act 2, Alfira must survive the Last Light raid. Lakrissa is in the Moonrise Towers prison group and must be broken out.
Act 3, both are present in the Elfsong Tavern for closure conversations: Lakrissa as a waitress and Alfira on the roof.
- Mol (primarily Acts 2 & 3)
Act 2, after the Last Light raid, speak with the other tiefling children to pick up the Find Mol quest.
Act 3, Mol can be found in the Guild headquarters for quest completion and closure.
- Bex & Danya (primarily Act 2)
Bex must survive the Last Light raid, and Danya must survive the Moonrise Towers prison break.
Speak with her to get the quest to find Zevlor. She must survive the raid on Last Light.
He should survive the initial combat triggered on discovering the Druid Grove.
In Act 2, find and release Zevlor from the Mindflayer Pod in the Mindflayer Colony, and make sure he survives the fight against the also-released mind flayers.
The following tieflings have received mixed reports as to whether or not they count; some reports claim they did and others say the achievement still triggered even without interacting with them at all:
- Nadira (Act 1) (saved in my playthrough)
The astronomer up on the hill near the gate in the Druid Grove. Coming too close triggers a proximity-based encounter with a bugbear assassin. Save her life.
(Notably, mixed reports here are frequently reports where the achievement triggered despite not being aware of the character's existence at all. Since the initial encounter for her seems to be on a proximity-based trigger, not getting close enough to trigger the encounter in the first place may count as 'saving her life'. This is not confirmed.)
The following tieflings are, by October 2023, generally confirmed not to count, despite early anecdotal reports including them. They may still be saved if you wish:
Act 1, save him from the harpies on the shore in the Druid's Grove.
Act 2, he can be found with other tiefling children in Last Light (if Rolan was convinced to stay in Act 1); speak to them after the attempted raid to talk them out of going after Mol themselves.
In Act 3, he can be found with Mattis by the refugee supplies warehouse in Rivington for closure conversation.