As per comics, she is really really old. But she isn't "from the past", she just lived through it.Is Agatha Harkness from 'our' time (modern day) could she be from the past hence why the items keep slipping back to the past?
She is also wearing a super-suit...One cool thing from the way this episode played out is that Darcy is now effectively a rogue element. With Vision having 'de-whammied' her and then headed off without her, she may well be the only person in the Hex aside from Agatha and possibly Vision himself with a mind of her own.
I thought that her eyes glowing showed that she was activating her own powers, like when she came through the barrier.I need to watch it again to confirm, but a lot of the talk online about this scene is that Monica's eyes supposedly went purple after she opened the cellar door and saw how weird it looked inside. So it may be saying she is under Agatha's control now, instead of Wanda's or instead of being free of any control. Maybe it is hinting at a Monica vs Vision fight or something?
Edit: Just watched that again and yes, her eyes turn purple right before Pietro surprises her and are still purple when she looks at him. So that means something important in the mind control area.