The Crimson Balmung is a reward for completing the miniquest Koschei's Troubles. It can be changed into the Achromatic Balmung by right clicking and selecting the "Toggle-colour" option.
When used on dagannoth, all enhanced Balmungs have a 45% bonus to hit chance, and deal 45% more auto-attack damage, or 35% against Dagannoth Supreme. With abilities, the extra damage is 22.5%, or 17.5% against Dagannoth Supreme.
If your Strength level is below 90, boosting your Strength will not allow you to obtain the jewel and the Crimson Balmung.