Aker is an earth god who also presided over the western and eastern boarders of the Underworld.
In early representations, Aker is shown as a narrow strip of land with a human or lion head at both ends But later he was shown as the foreparts of two opposing lions, sometimes with human heads, facing away from each other. One lion faces west while the other faces east. In between them is the sign of the horizon. In the later period of Egyptian theology the two lions making up the Akeru were named Sef and Tuau - 'yesterday' and 'today' respectively.
Ancient Egyptian mythologists believed that during the night the sun journeyed through a tunnel that existed in the earth - its entry into the tunnel caused the night, its emergence again bringing the day once more. Each end of this tunnel was guarded by a lion god.
It was Aker who opened the earth's gate for the king to pass into the Underworld. He was also known to absorb the poison from the body of anyone bitten by a snake and he neutralizes the venom in the belly of a person who has swallowed an obnoxious fly.
More importantly, he imprisons the coils of the snake, Apophis, after it is hacked to pieces by Isis, and Aker could, along his back, provide a secure passage for the sun-god's boat as it traveled from west to east during the hours of the night.
From the tomb of Ramesses VI in the Valley of the Kings on the West Bank of Luxor (ancient Thebes), the tomb of Pedamenopet (26th Dynasty) at el-Asasif, also on the West bank at Luxor, and mythological papyri of the priesthood of Amun in the 21st Dynasty, it is even possible to reconstruct a "Book of Aker", concerned with the solar journey.
There was also a more threatening side to Aker that can be seen when he is pluralized as Akeru in the form of multiple earth gods. In passages from the Pyramid Texts, the Akeru are said not to seize the monarch, but later there is a general hope for everyone to escape the grasp of the earth gods. The Akeru appear to be primeval deities more ancient then Geb.
In Egyptian mythology, Aker (also spelt Akar) was one of the earliest gods worshipped, and was the deification of the horizon. There are strong indications that Aker was worshipped before other known Egyptian gods of the earth, such as Geb. In particular, the Pyramid texts make a sinister statement that the Akeru (plural of Aker) will not seize the pharaoh, as if this were something that might have happened, and was something of which to be afraid. Aker itself translates as (one who) bends, and thus Akeru translates as benders, though in what sense this is meant is not fully understood.
As the horizon, Aker was also seen as symbolic of the borders between each day, and so was originally depicted as a narrow strip of land (i.e. a horizon), with heads on either side, facing away from one another, a symbol of borders. Since the sun reaches its peak (its solstice) in the zodiac of Leo, these heads were usually those of lions. Over time, the heads became full figures of lions (still facing away from each other), one representing the concept of yesterday (Sef in Egyptian), and the other the concept of tomorrow (Duau in Egyptian).
Consequently, Aker often became referred to as Ruti, the Egyptian word meaning two lions. Between them would often appear the hieroglyph for horizon, which was the sun's disc placed between two mountains. Sometimes the lions were depicted as being covered with leopard-like spots, leading some to think it a depiction of the extinct Barbary lion, which, unlike African species, had a spotted coat.
Since the horizon was where night became day, Aker was said to guard the entrance and exit to the underworld, opening them for the sun to pass through during the night. As the guard, it was said that the dead had to request Aker to open the underworld's gates, so that they might enter. Also, as all who had died had to pass Aker, it was said that Aker annulled the causes of death, such as extracting the poison from any snakes that had bitten the deceased, or from any scorpions that had stung them.
As the Egyptians believed that the gates of the morning and evening were guarded by Aker, they sometimes placed twin statues of lions at the doors of their palaces and tombs. This was to guard the households and tombs from evil spirits and other malevolent beings. This practice was adopted by the Greeks and Romans, and is still unknowingly followed by some today. Unlike most of the other Egyptian deities, the worship of Aker remained popular well into the Greco-Roman era. Aker had no temples of his own like the main gods in the Egyptian religion, since he was more connected to the primeval concepts of the very old earth powers.