The Spear Forge lets you create a powerful spear to attack your opponent or the summoning orb.
Equipping the artifact will allow you to summon a mystical anvil, on which you can forge a mystical javelin which can unleash a powerful magical attack that can breach through shields upon impact. To forge the magical javelin, you must strike Spear Forge on the precise spot (marked by magical sigil) three times consecutively. Succeeding in doing so will result in the anvil disappearing, leaving the complete magical spear in the place of the hammer in your hand, ready to be thrown. It will fly in a straight line, ignoring gravity.
This looks like an extra large fireball when it's coming at you. You can't see the spear itself.
If someone is forging the spear hit them for free with your main hand and prepare to teleport with your shield hand, but don't teleport right away if you can help it. Wait till they throw it and immediately teleport to avoid about 30% damage.