Sukunas cursed technique is probably an assortment of different abilities based on different types of weapons/tools. As shown by the image above Sukuna's two slashing attacks are based on different knives.
The mythological Ryomen-Sukuna that Sukuna from jjk was based on is also known to wield many weapons. In the coloured page we see Sukuna in what we can presume to be his true form wielding two different weapons, whilst in the manga panel we see sukuna form an arrow of flames
This variety of weaponry all points towards his cursed technique being about his weaponry arsenal
It's also likely that Sukuna stores these weapons in his tattoos since before he uses his flames he says "open" and an image of a black block is also shown which is reminiscent of the tattoos on his neck
In sukunas true form he's holding two weapons and if you look at his arms he has four single armband tattoos. On the left we see Sukuna with no weaponry and instead of four single armband tattoos they're in pairs. The loss of tattoos could signify the weapons being released
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