God of War Ragnarok might've quelled a lot of questions from the previous installment, but the reason behind Kratos' arrival at Midgard remains unanswered. The series' shift from Greek mythology to Norse, although a bit jarring for veteran God of War fans, was intentional.
Developer Santa Monica Studio wanted to refresh the series with a soft-reboot, giving players a brand new mythological setting to explore while keeping Kratos' character intact.
With God of War Ragnarok being the final chapter in Kratos' Norse saga, fans were expecting to get a definite insight into his arrival in the Norse lands. However, there isn't one.
While there are a few hints throughout the story, which culminate towards a somewhat definite answer, nothing is concrete.
There is also a tie-in novella for 2018's God of War, written by J.M. Barlog, that sheds some light on how Kratos might've made his way to the Norse realms. However, it is also only surface-level information and nothing concrete.
Although neither God of War Ragnarok nor God of War (2018) specifically answers the question, there are plenty of clues to work with.
Note: This article contains spoilers for God of War Ragnarok and God of War (2018).
Kratos' arrival in Midgard is a huge mystery in God of War Ragnarok
Kratos' arrival in the official novelization, side stories, and optional conversations in God of War Ragnarok
In the official novelization of God of War (2018), it was hinted that after the fall of Greece in God of War 3, Kratos traveled for days on end, without any definite destination. The former God of War seemingly sacrificed his godhood to restore "hope," and was cursed with immortality.
Thus, despite impaling himself through the chest with the god-killing "Blade of Olympus," Kratos survived. However, he found himself in an entirely new and unknown realm, greeted by a fair-haired warrior wielding a frost-imbued ax.
Kratos being the rage-fueled warrior he is, immediately starts charging towards the warrior, only to be matched in strength and combat skills.
The warrior was later revealed to be none other than Faye, who after a fierce battle with Kratos, took him under her care, giving him a place and a newfound purpose to live. This sequence of events was even mentioned in God of War Ragnarok in a few flashback sequences, where Kratos reminisces over Faye's memories.
The mysterious "rift between the worlds" found by Odin might be linked to Kratos' arrival in Midgard
While not directly related to Kratos, Odin was seen obsessed over a mysterious "rift between the worlds," which can be a massive hint towards Kratos' arrival in the Norse realms.
While nothing much is explained about the "rift," Odin considers it to be a place with infinite knowledge over the past, present, and future. Furthermore, it might be something he plans to harness to avert Ragnarok. The "rift between the worlds" can very well be a multi-versal portal that first opened up when Kratos arrived.
After the phenomenal reception of God of War (2018), Cory Barlog, the creative director at Santa Monica Studio, was asked about Kratos' entry into the Norse lands. He was also asked how two vastly distinct mythological pantheons with distinct rules of creation and existence can "exist" on the same plane.
Barlog replied that both the Greek and Norse settings in the God of War games are free from the rules, regulations, and constraints of one another. While the answer is vague, it sort of implies that the Greek pantheon and the Norse realms are two distinct realities, perhaps akin to a multiverse.
However, the multi-versal theory is a bit farfetched, since there isn't concrete evidence of multiple universes co-existing within the overarching narrative of the God of War series. This is besides the "rift between worlds" that Odin obsessed over.
Kratos' arrival in Midgard can be attributed to a narrative tool
The final reason for Kratos' arrival in Midgard can be attributed to the developers just wanting to introduce his rage-fueled character to a completely new setting.
This was also the only logical solution to continuing the series, given the Greek pantheon was all but destroyed at the end of God of War 3.
While there is no conclusive answer to how the Ghost of Sparta made his way to an entirely different mythological setting, fans aren't complaining. Kratos' arrival in the Norse realms is what allowed developer Santa Monica Studio to flex their creative muscles in such interesting ways.
To everyone's delight, this resulted in the creation of two of the best action-adventure games out there, God of War (2018) and its sequel, God of War Ragnarok.