Final answer:
Defeating the celestial invasion in Terraria summons The Moon Lord, the final game boss. Players must prepare with strong equipment and potions for this challenging battle.
Step-by-step explanation:
Defeating the celestial invasion in the game known as Terraria summons C) The Moon Lord, which is the final and most powerful boss of the game. Upon defeating the four celestial pillars, the Moon Lord will be summoned after a short in-game period. The Moon Lord presents a challenging fight, with multiple phases and devastating attacks. It is crucial for players to be well-prepared with strong weapons, armor, and potions to stand a chance against this formidable enemy.