Core is the 8th and final level of Celeste. It is something of a bonus level and is not played at all in any%. It has a large number of new mechanics which appear nowhere else. Firstly, Madeline has two dashes, like in The Summit, but no longer recovers them when touching the ground. Instead, the only ways to recover dashes for most of the level are screen transitions and refresh gems. Secondly, many rooms have "red" and "blue" variants, which can be swapped between with the use of switches and which alter the properties of some of the objects in the level. The level always starts and usually also ends in red mode. Thirdly, there are fire/ice blocks, which, when grabbed or stood on, give Madeline a boost in the direction she came from in red mode and sink slightly without doing anything exciting in blue mode, disappearing shortly afterwards in either case. Fourthly, some levels are mostly vertical and feature rising floors of death. Others are more horizontal, and feature both floors and ceilings of death, which slowly rise in red mode and slowly fall in blue mode. Fifthly, there are "zip walls" which, when grabbed, accelerate Madeline upwards in red mode but which cannot be grabbed at all in blue mode. Sixthly, there are fireballs, which are always deadly in red mode but can be bounced on from above in blue mode. Finally, the final rooms of both 8A and 8B are "space rooms", in which all of Madeline's physics is somewhat slower than normal, the dash recovery restrictions are gone (returning Madeline to The Summit rules) and in which one can loop from the top of the room to the bottom and vice versa. In 8B, the space room doubles up as the tape room.
Core has gates which block progress unless the player has at least some specified number of hearts. 8A only needs 4 hearts out of the 14 available, while 8B and 8C require 15 and 23 hearts respectively, which are the maximum numbers of hearts achievable up to that point.
8A contains "fireskip", a major skip where a significant section of the level can be bypassed by moving between two counter-rotating strings of fireballs while still in red mode instead of changing to blue mode, which makes this particular obstacle much easier. This can be extended to "iceskip", which is used to obtain a berry in a way which also skips that section in full clear runs. This trick, in addition to the fact that 8A contains only five berries (the second fewest behind Reflection A) and all other collectibles are mandatory, means that the time difference between 8A clear and full clear runs is much smaller than for any other level.
In both 8A and 8B, there are instances where one has a choice of completing levels in either red mode or blue mode, but, usually, one path is clearly intended. In 8B, breaking from the intended path saves time in at least two places. Playing the first two rooms of the fourth checkpoint in blue mode instead of red mode, known as "heartskip", saves about 5 seconds, although it has a number of variants which can alter this number. Playing the last two rooms of the level in blue mode is a more aggressive strat called "blueskip", which saves about 1 second. This causes the level to end in blue mode, which affects the tape room's appearance, although the level functions the same as it always does.
In 8C, the heart gate which halts progress unless the player has 23 hearts is in the first room, instead of the third or second rooms in 8A and 8B. While one normally needs to wait for the gate to open, in 8C, one can open the gate, then suicide. Because Madeline has not yet left the first room, the timer resets when she respawns. This trick allows one to start with the gate open, saving roughly 5.5 seconds. 8C also has the mythical "TAS strat" in which one obtains great speed at the beginning of the fourth and final room, which is extremely long, and maintains this through a large proportion of it. This has never been done RTA, but is believed it will eventually become viable. While 7A and 7B are the longest A- and B-Sides, 8C is the longest C-Side. Even so, it is still slightly shorter than the shortest non-C-Side, which is Forsaken City A.
At the time of writing, the longest running Celeste record ever was Core A 4:07 by Glackum, set on 16 April 2018, which stood for 193 days before being slain by Aurora Dash with Core A 4:04.
Clear IL WR Progression (8A)
B-Side IL WR Progression (8B)
C-Side IL WR Progression (8C)
Note: Spirialis' Core C 55 was retracted on 5 September 2018, despite the fact that its legitimacy was not in question. This caused the WR to revert back to Lvl19's 8C 56.
Full Clear IL WR Progression