Skyrim SE version here: http://www.nexusmods.com/skyrimspecialedition/mods/3403

HEARTHFIRE required, obviously.

Voice credit: Thanks to Esteris here on Nexusmods for recording some lines for this mod!

--- Version 2.x ---

Mod to allow player to adopt Helgi the ghost child.

After talking to the Jarl to collect the rewards for completing the quest Laid to Rest, go to the burned house in Morthal. You'll find Helgi there, and she will be adoptable.

Helgi sometimes takes a long time to move from Morthal to her new house. Try traveling between faraway map locations to pass the time, and eventually she should be at your house.

If you're like me and have finished Laid to Rest a long time ago, but still want to adopt Helgi, you can get Helgi to appear at the burned house by using the console: startquest ms14helgiadopt
Obviously, don't use this console command if you are NOT yet done Laid to Rest, otherwise you'll mess up your game.

IMPORTANT NOTE: Once Helgi is adopted the game treats her just like any other adopted kid. To unadopt Helgi you can't just disable this mod; you'll need to do stuff that allows you to unadopt any kid, such as disabling Hearthfire entirely and then re-enabling it again.

- Mod Compatibility Info -

Mod compatibility should, for the most part, not be a problem. Tested with USLEEP, Hearthfire Multiple Adoption, RS Children, The Kids Are Alright, Dolls. Mods that change Helgi's appearance have slight conflict with this mod. Adopting Helgi is not a problem where compatibility is concerned, it's just her appearance (e.g. custom clothing) and/or her name that's affected, depending on your load order.

If you use RS Children, download the optional file "Adopt Helgi Ghost - RS Children" and load it below both RS Children and Adopt Helgi Ghost.

If you use The Kids Are Alright, download the optional file "Adopt Helgi Ghost - TKAA" and load it below both The Kids Are Alright and Adopt Helgi Ghost.

If you use Dolls, download the optional file "Adopt Helgi Ghost - Dolls" and load it below both Dolls and Adopt Helgi Ghost.

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Outdated Version 1.0 Description

This mod is only useful for people who have save files where the Morthal quest Laid to Rest hasn't been completed yet. After clearing Movarth's Lair and talking to the Jarl, go back to Movarth's Lair and talk to Helgi to offer to adopt her. 

As far as I can tell, this mod is compatible with USKP, USLEEP, Hearthfire Multiple Adoption, RS Children. Should load this mod after all of them.

Sometimes Helgi will appear to have adopted - she'll says "thanks" and "I'll meet you at home" - but for some reason does not leave Movarth's Lair. It might be the game itself having errors with paths or something. Try reloading the game to before you offered to adopt her, and offer to adopt her again. If that still doesn't work maybe try using the console to move Helgi to your house.

Steam Workshop: http://steamcommunity.com/sharedfiles/filedetails/?id=476793695