I wouldn't say that this Card is in any way underpowered, compared with Lord of Time. Lord of Time is, like Arthoria the Saber, durable and swift. Arthoria got also the banish keyword, Lord of Time got the Mage Keyword. This Card deals in fact more damage than Lord of Time, with 40 after the second playing of this Card, and with this, i would say that this Card is more effective than Lord of Time, since this Card got a reduction of Cost for the first playing of this Card, but less damage.
So with this, this Card is, compared with Lord of Time, the way that it is well balanced and don't need the Legend Keyword, since this or the remove of the Enduring Keyword would destory the idea of this Card.
(An Legend/Durable/Swift Card would be overpowered anyway, the cost does almost not matter)