That is some variety or another of a “torx star” head. They vary in size and are listed as TS-10, TS-15, etc. your best bet would be searching torx star and buying a set of bits to try out various sizes.

Pick up the IFix it screw set, it has socket style bits that will fit that, plus most of the goofy screws in electronics, I'm a broadcast engineer and that set is awesome for work and hobby electronics work.

Edit: I just looked at their site and seems like they've grown a bit in the last ten years, they have many sets but here's a link, they definitely have what you need https://www.ifixit.com/Tools

Edit again: I was wrong. TIL about EPR Torx. Just search that and you should find the right set.

Hard to say without a scale reference, but if it's small, I very small might very will be a proprietary fastener for higher security applications. If it's a Tesla battery pack, then the epr 5-point torx is a good candidate. If it's an apple device then searching pentalobe might get you farther.

What on earth is it? Pics? Ruler next to it?

Idk but buy a safety screw kit

It's hard to tell from the pic, but while that may be epr torx, it also looks like a pentalobe.

Screw iou

Torx