Actually, that indicator isn't indicative of Events at all; rather, it represents your current "Land Strength" based on the number of Artifacts you've placed. As you place more Artifacts, the indicator will grow.

In order to activate "The Cage of Dreams" you need to reach the second-to-last 'growth level' of that indicator as it is, so it's quite easy to reach the final 'growth level' before actually proceeding into the Tree of Mana.

As a small note, it's entirely possible to complete every Event in the game before heading to the Tree of Mana (and thereupon start "The Legend of Mana" Event), though it does require some planning.