PARTICLE* p = ent_decal(you,bmMark,7+random(3),random(360)); // place a random sized decal at the hit entity 
            p.lifespan = 1600;   // remove decal after 100 seconds
            p.material = mat_model; // assign a dedal material

void _testevent (PARTICLE* p){
   p.alpha -= time_step;
   if (p.alpha <= 0) p.lifespan = 0;
 //p.z -= time_step * 2; // wont work too  
}
...
   PARTICLE* p = ent_decal (you, bmMark, 7+random(3), random(360));
   p.material  = mat_model; // mat_particle, mat_sprites doesnt matter
   p.alpha     = 80;
   p.vel_z     =  3;
   p.gravity   =  3;
   p.event     = _testevent;
   set (p, MOVE | TRANSLUCENT);
   /*//and this wont do it also
   while (p && p.z > 0){
      p.z -= time_step * 2;
      wait (1);
   }*/
...