Village of the Cactuars is a side quest in Final Fantasy X which allows you to get Rikku's Sigil and also some treasures.
The aim of this side quest is to find each cactuar and then play a sub-mini game. You will have to reach the Cactuar without it looking and before the timer runs out (like "Red Light, Green Light"). You will then have to battle it, (use Wakka as he has more accuracy). Doing so will get you a named sphere, if it sees you or you run out of time you will get a "Sphere del Perdedor" (loser sphere). Getting more named spheres will get you better prizes, however it isn't necessary for you to get the named spheres.
There are 10 Cactuars in all and you need to find them one by one. To initiate the mini-game go to the fourth area and examine the Cactuar glyph and you will be given a clue to where the cactuar is.
The following is a list of all the Cactuars and where to find them:
After putting nine spheres into the rock, the sandstorm will stop. Search the area for two treasure chests. One will contain the Mercury Sigil. The other will contain a prize depending on how many named spheres you collected: