The event is live until April 7. You can log in daily to receive a special item. After this, complete dungeons which will net you the T-E-R-A letters. Once you have all four letters, combine these to create a Forgeheart Key. Using this key, you can unlock Forgeheart Jackpot boxes.
These boxes can contain Annihilation or Dark Light Weapons for your specific class. Additionally, note that during the first week of the event (until March 23), logging in daily will give you an Infinity Coin which cancels out the Adventure Coin cost for any dungeon for the next hour.
The second week of the event will give you a special enchanting scroll as part of your daily login. You can use these on superior-rarity Exodor gear. Oh, and about those letters?
“Every level 65 dungeon you complete has a chance to drop a Forgeheart Letter Box with a T, E, R, or A inside. And if you find yourself with too many of one letter and not enough of others, you can dismantle letters into Forgeheart Tokens exchangeable for the letter of your choice through Forgeheart Envoys in Velika and Highwatch.”