The Sekhmet Costume granting Bayek darker robes and a metallic mask in the image of the lioness goddess is easier to get, and earlier. And dare we say, might even be preferred to the Isu Armor by many players. The only real obstacle to avoid is to not be in a hurry to head to Alexandria when receiving the Aya Quest. Instead, take the detour to the smaller settlement of Yamu where the quest titled Ambush in The Temple can be unlocked. Once completed, Bayek will be prepared to take a much more intimate role in the celebration of Sekhmet about to begin in Yamu. Completing the quest allows players to unlock the Ulterior Votive, battling bandits to help out Mehenet.
It shouldn't pose too much of a challenge, but the real trick comes when the fighter set to play Sekhmet in the ceremonial battle is... indisposed. Fetch his unconscious body from the tavern straight outside of Yamu's front entrance, and Mehenet will allow Bayek to take his place. Don the Sekhmet Costume as her avatar in the battle against the representative of Isfet, and emerge victorious, and your job is done. Once the final cutscene of the mission plays, the Sekhmet Costume will be added to the player's inventory.
First things first: you can't unlock the outfit transforming Bayek into the form of Anubis - not yet anyway. The costume was revealed and vaguely described by developers for the obvious wow factor. The jackal headpiece, the flaming arms and weapons, and general terror it's guaranteed to instill in enemies is obvious. But to unlock this costume means doing so piece by piece, entering into the most grueling gauntlet the developers have created in the game's release schedule. Prepare for the Trial of The Gods.
It's a free activity still coming for Origins players, which will pit them against the literal forms of the Egyptian gods (the giant Anubis and respective soldiers visible in the image above). Completing a single Trial grants a single weapon, and only when all four have been gathered is the full outfit unlocked. If it sounds like an impossible task, it should: the developers intend to make any player seen with this outfit among the most respected in the community. So... the Sekhmet and Isu looks may have to do.
For more details to help you track down the Isu Armor, check out the Stone Circle locations (thanks to JorRaptor).
Assassin's Creed: Origins is available now on PC, PS4 and Xbox One.