Rin teaches you basic rules

Rin said: "Since you make your decision, I must teach you some basic things.

There are seven Masters who fight in this war. A Master will lose by two occasions: she is killed, or she gives birth. The last one left is the winner. Every means is acceptable: magic, weapon, poison..."

"In that case, can we use sneak attack? I think that is the simplest way."

"It's not that easy." Rin pointed to the mark on her belly: "This is the Command Spells. Each Master gains three of it when forming a contract with their Servant. It allows Masters to control their Servant regardless of distance."

"Isn't the Servant always follow your order?"

"Sometimes a Servant, especially a Berserker, didn't listen to orders from their Master. The Command Spell permits the Masters to issue an irrevocable and absolute order to the Servant. It also permits the Servant to accomplish an incredible feat, draws their power to beyond the limit."

"Is it too convenient?"

"No, the magic always comes with a price: Each time a Master used a Command Spell, the fetus will grow by 3 months. As the Servant will disappear when their Master gives birth, using Command Spell must be the last resort."

Seeing the worry in your eyes, she laughed: "I mastered Labor-Repressing Spell, so I still can use all three Command Spells. Of course, giving birth to an overdue baby must be painful."