I'll be honest, regular 2D chess was already enough to sizzle my brain - but apparently that wasn't difficult enough, as someone has created 5D chess.
Released last week by Conor Petersen and Thunkspace, 5D Chess claims to be "the first ever chess variant with spatial, temporal, and parallel dimensions". There's multiverse time travel, branching timelines, parallel dimensions - all something that makes it sound more suitable for Dr. Who than the average human.
Early Steam and YouTube reviews have argued it's actually closer to 4D chess, as the game features two physical and two temporal axes ("height" is not interactable). The explainer by MariAurum below finally helped me gain an understanding of the four axes: there's your regular x and y physical axes (left, right, up and down on a traditional chess board), and then there's your temporal axes, one that represents "time" (as in the past and future of a board), and the second representing parallel dimensions.
According to an explainer released by Thunkspace, pieces have specific rules for how they can move across time and space. Pawns, for instance, can only timeline hop "in the direction they can normally move on their board", while rooks can "time travel to any board so long as the position they are currently occupying is not taken, but they cannot move while time travelling". Simple.
While researching this article I also discovered that three-dimensional Chess is actually a thing and not just a turn of phrase (or made-up Star Trek game). The difference here is that pieces can move in three physical dimensions, with essentially stacking boards. German chess master Lionel Kierseritzky is credited with coming up with the first three-dimensional chess design back in 1851 (via Geek & Sundry).
Even if all this sounds like too much for you, it's at least worth reading some of the Steam reviews for the pure absurdity of how 5D Chess games play out.
"I was playing a game against a human opponent online, and at one point they sent a queen back in time from one of the ten timelines currently in play to put five of my past kings into check at once", said TheSpookiestUser. "I sent one of my own pieces even further back to stall, and they proceeded to send one of their queens back to the start of the game to try and beat me before I even got to that point.
"I was able to manoeuvre one of my bishops in the second-most divergent timeline into position to capture the queen in the alternate present once we got back to that point and save the game (I eventually won by checkmating their king 5 turns in the past). This is an actual description of an actual game."
Steam user jolemo, meanwhile, noted that "there are few things in life more satisfying than being able to declare checkmate upon your opponent throughout all of time and space". I did actually manage to find a video of someone achieving a triple checkmate across multiple dimensions.
Although some user reviews have critiqued the AI behaviour, it's also possible to play online if you want to test your mettle against other, equally-confused players. You can try 5D Chess out for yourself for a discount price of £7.43 on Steam, until that offer ends on 29th July. Unless you've also figured out real-life time travel.