This achievement is for completing chapter 4 and defeating/freeing Illtauga the giantess. It unlocks after the continuous cutscene end and you gain control at the start of chapter 5.
The giantess sequence is simple enough where you have to run and get into the shadow of the next stones when Lava/flash rises.
03:24:55 - Chapter Five - Bárðarvik
03:25:30 - To The Sea
03:26:25 - "Returned to the Earth" Achievement
Extended Timestamps:
00:00:00 - Intro & Recap
00:06:00 - Chapter One - Reykjanestá
00:11:00 - Landfall
00:15:45 - "Worth a Thousand Words" Achievement
00:16:45 - 01/18 Lorestangir
00:22:05 - Shore of Slaves
00:26:15 - Capsized Ship
00:31:10 - Slavemaster Found
00:37:00 - "The Next Sign" Achievement
00:37:10 - Chapter Two - Freyslaug
00:38:20 - Freyslaug Sighted
00:39:00 - 02/18 Lorestangir
00:46:15 - Settlement Entrance
00:49:50 - Return Home
00:55:25 - 03/18 Lorestangir
00:56:40 - 01/17 Hidden Face
00:59:20 - Draugar Ceremony
01:04:35 - 04/18 Lorestangir
01:08:40 - Draugar Battle
01:11:35 - Meeting the Stranger
01:13:55 - "The Man in the Middle" Achievement
01:14:30 - 02/17 Hidden Face
01:16:45 - 05/18 Lorestangir
01:21:20 - Chapter Three - Rauðhólar
01:22:00 - Red Hill
01:22:45 - 03/17 Hidden Face
01:24:10 - 04/17 Hidden Face
01:25:20 - 06/18 Lorestangir
01:28:00 - Illtauga
01:32:20 - On the Hill
01:44:00 - Find the Hiddenfolk
01:47:55 - 07/18 Lorestangir
01:50:15 - 05/17 Hidden Face
01:53:00 - 06/17 Hidden Face
01:54:20 - 08/18 Lorestangir
01:59:15 - 07/17 Hidden Face
02:00:25 - 09/18 Lorestangir
02:01:40 - 10/18 Lorestangir
02:05:05 - 11/18 Lorestangir
02:10:40 - Chapter Four - Huldufólk
02:11:15 - "The Hidden World" Achievement
02:11:40 - Enter the Caves
02:12:55 - 08/17 Hidden Face
02:14:35 - 12/18 Lorestangir
02:16:45 - 09/17 Hidden Face
02:18:25 - 10/17 Hidden Face
02:22:45 - 13/18 Lorestangir
02:30:55 - Trial of Wisdom
02:37:25 - Into Darkness
02:43:20 - Act of Sacrifice
02:49:10 - 11/17 Hidden Face
02:50:35 - 14/18 Lorestangir
02:57:15 - Breaking Through
03:05:15 - A Gift Returned
03:15:20 - The First Ritual
03:24:55 - Chapter Five - Bárðarvik
03:25:30 - To The Sea
03:26:25 - "Returned to the Earth" Achievement
03:27:25 - 12/17 Hidden Face
03:28:40 - 15/18 Lorestangir
03:30:25 - Sjávarrisi
03:35:00 - 13/17 Hidden Face
03:39:00 - 16/18 Lorestangir
03:47:10 - 14/17 Hidden Face
03:49:00 - Another Question
03:53:05 - 17/18 Lorestangir
03:54:40 - 15/17 Hidden Face
04:05:25 - 16/17 Hidden Face
04:06:55 - 18/18 Lorestangir
04:07:30 - "Tales of Midgard" Achievement
04:08:10 - 17/17 Hidden Face
04:08:25 - "Glimpses of the Gods" Achievement
04:10:00 - Ástriðr’s Story
04:22:50 - Waking the Giant
04:24:40 - The Second Ritual
04:37:50 - Chapter Six - Borgarvirki
04:38:45 - Into the Forest
04:38:50 - "The Storm Calmed" Achievement
04:52:30 - Borgarvirki
04:55:00 - "A Guiding Light" Achievement
04:58:30 - Heart of Darkness
05:14:50 - The Last Lie
05:23:15 - Ending
05:27:45 - "End the Tyranny" Achievement
05:28:15 - "Seeing Things Differently" Achievement
05:28:50 - Subscribe!