I remember a Kamen Rider who was transformed from the mark on the back of his hand, and this mark appeared randomly. I remember he seemed to summon armor or something.

I think it's the superstar Granthersa.

The power of the twelve constellations has been handed down from ultra-ancient times. These twelve forces are transformed into twelve marks on the back of the hand of the person who inherits the power, which can be transformed into soldiers with constellation power. They are called Gransaisha, and the twelve Gransaisha are divided into four races: wind, water, earth and fire. There are three people in each race. When three Granisesas of the same race arrive, they can summon superstars belonging to their race.

The four superstars are divided into the wind-like star god Doru Kuruz, the water-like star god Bai, the earth-like star god Gonsisa, the fire-like star god and the later djinn-like star god Gunter Lars. Five superstars can be combined into a combined beast, Sai Sand King.