The heartseeker is the basic AD item and is powerful because it gives 25 physical power, 10% movement increase, and stacks for "+1.5 Physical Power per stack, and receive 5 stacks per player kill and 1 stack per minion kill. All stacks are lost on death (max 60 stacks)"
at Level III. Most likely you will be soloing mid as Ra, since he clears lanes like they are NOTHING. In addition, your mid opponent will most likely be a magical damage dealer such as Ao Kuang or Agni. If you can hit your auto attacks will deal massive damage to magical dealers, since their weakness is physical damage. Finally, your magical damage from your skills will also deal a lot of damage dearly game.
Fatalis isn't just for attack damage characters, as it increases the attack rate significantly for all gods. Fatalis at Level III gives 10% movement speed, 35% attack speed, and ability cooldown is reduced by 20%. Also, fatalis is great for Ra because Ra's attack speed is pretty slow. (It's rather frustrating to auto-attack when your attack speed is very low)