Have a Horse Friendship Scroll, obtained via the Horse Friendship Scroll Quest, and say Horse Friendship Scrolls to Malachai Thanatos, the Centaur Archer, located in the Centaur Town.

Reward:

• 9,000 XP per level

And, randomly one of the following:

• Sinister Warblade
• Assassin's Blade
• Centaur Slayer Bow
• Hellish Robe

You can complete this quest at will.