Sorry to be the one to tell you this, but nobody will give you Undertale for Darkness Within.
Darkness Within is a game that can be purchased for $1 in the weekly HB while Undertale is a relatvely new game that hasn't been bundled and is currently $10.
I can give you something from here for Darkness Within:
Starseed Pilgrim gift
grid 2 gift
Half Minute Hero: Super Mega Neo Climax Ultimate Boy gift
Command & Conquer: Red Alert 3 - Uprising Steam Key
Medal of Honor (2010) Steam Key
NiGHTS Into Dreams gift
Batman™: Arkham Asylum GOTY gift
Trine 2 Complete Story gift
Warhammer® 40,000: Dawn of War® - Game of the Year Edition gift
King's Bounty: Armored Princess gift
Dwarfs!? gift
Zeno Clash gift
Red Orchestra gift
The Last Remnant gift
Tomb Raider I gift
Tomb Raider II gift
Tomb Raider II gift
Far Cry 3 Blood Dragon Uplay key
mihaipuiu, I'm interested in your offer. I want Tomb Raider, but I can't choose which one. Is there an option to get TB 1, 2 and 3? If not, you can choose for me which TB I'll get, since I can't choose. :D
Darkness Within 1: In Pursuit of Loath Nolder + OST
Undertale
This is my first trade at SteamGifts.com, and the reason I'd like to sell this game is that I bought a Humble Bundle with this game. Turns out it's Darkness Within 1, a horror game, and I'm such a fan of horror games! sarcasm If I weren't right, the key should also give you the OST DLC with it, because other games I've bought one HumbleBundle.com gave me at other game also all DLC available. If not, we can figure it out, ex. trading stuff. And I want Undertale soooooo badly, that I'm filled with determination, so I hope you guys will help me out. =D