Make sure to complete the Omen three times, the first time in 1000 AD, the second in 600 AD, and the third in 12000 BC (if you complete it in an earlier time period, you won't be able to complete it again in a future era). Also make sure to Charm the Prism gear from Zeal's arms each time so you can have as many pieces of Prism equipment you can get in a single playthrough.

If you go to the Omen in 2300 AD, you can't enter but you can get a Tab if I recall. But it's been awhile.