As the giant comet approaches, the Earth becomes what might be described as confused. There are conflicting pulls, gravitational as well as magnetic. The Earth is normally magnetically aligned, along with the other planets in the Solar System, according to an axis that exists in your part of the galaxy. This axis reflects the sum of a number of solar systems and other matter not understood yet by your scientists, but is normal and natural for your part of the galaxy. Now comes the giant comet, from a distance, swinging through your Solar System, and picking up speed as it approaches. It outweighs the Earth by some 23 times, but by size is not 23 times as large as its weight is due to its makeup, having less silicon than the Earth, for instance. Should one line these planets up side by side, the Earth would look to the 12th Planet as the Moon does to the Earth. Where the gravitational pull on the surface of the 12th Planet is better than half again as much as on the surface of the Earth, accounting for the stocky appearance of the large hominoids who live there, in the tug of war regarding Magnetic Alignment the 12th Planet's mass gives it a 4-to-1 advantage. There is no contest.
At first, your planet Earth reacts by additional swirling about in matter deep within its core. In the final days, as the giant comet begins passing between the Earth and its Sun, total interference occurs. The Earth slows in its rotation, and actually stops. The Earth's rotation is due to the balanced gravitational pull of the neighborhood, and the 12th Planet is a very big bully. The part of the Earth facing this bully is loath to leave, essentially, and this causes the rotation to gradually stop. This is recorded in written history and spoken folklore, worldwide, as a long day or night. The period of time, due to the lack of mechanical clocks, was not measurable, but the humans who experienced this described this as anywhere from a few hours to several days. The latter is more correct. Rotation also stops because the magnetic influence of the giant comet, which is aligned according to the larger Universe where it spends most of its cycle, has essentially gripped the Earth. The Earth has not yet joined the alignment of the giant comet, but what was occurring before, the steady, slow roll, has by now stopped. To make the switch, and magnetically align with the giant comet, is much more of an adjustment than simply slowing and stopping rotation. Thus this slowing of rotation occurs with little trauma.
Planet X grips the Earth, causing rotation stoppage, well before being between the Earth and Sun. It does so because it is approaching from the south, it's north pole to the Earth's south pole, at an angle such that the side of the Earth facing the Sun is more affected by the approach. Thus, while at some distance from the point of passage, it has gripped Earth to the point of not letting the highly magnetized Atlantic Rift go. During this week, the size of the approaching Planet X horrifies those on the daylight side of the Earth. At first a reddish blob only a fraction of the size of the Moon, it grows in size and appears to twist and turn like a dragon in the sky. The ancients recorded this as a fire dragon in the sky, as the tail of swirling moons gave this appearance. When approaching the point of passage, the shift suddenly happens, and this is before the point when Planet X is actually between the Earth and Sun. After the passage, Planet X exits as quickly as it approached.