The SIL ("Special Infinity Level", possibly) is a special level generated when the PC reaches the 66th level of the Infinite Dungeon for the first time. All items, walls, floors and doors in the SIL change colors at every turn, similar to how the terrain at the top of the Tower of Eternal Flames flashes orange and red.
Note that once the PC has left the SIL, they can never return to it again if they re-enter the 66th level of the ID. If they haven't collected the Sceptre, the player cannot possibly achieve an Ordinary chaos god ending; the game alerts the player to this with the message "You feel you leave an essential part of Chaos behind...forever". It is, however, still possible to achieve an Ultra Ending without the Sceptre of Chaos.