What Does Fallen Oni Do?
When you play this card, the top unit in your opponent's deck is discarded and sent to their void. That effectively means that you've pre-emptively killed off a random unit, regardless of how strong it is. This makes Fallen Oni a good choice for a Kleptomaniac deck build.
There's obviously a degree of RNG in how effective this card is. If you get lucky it can totally ruin your opponent's plan. That's especially true if you get the timing right against a deck build based around buffing units in the deck with Warcry.
What about Aegis? How does that interact with Fallen Oni? It appears that an Aegis on the unit makes no difference since the effect is being applied to the player - hence the wording "The enemy player discards...". However an Aegis on the enemy player's main character will prevent the effect working.
It's interesting to compare this card with Pilfer, another one cost Shadow card. With Pilfer you might get any card whereas Fallen Oni specifically targets a unit - that might or might no be an advantage. With Pilfer, unlike Fallen Oni, you get to keep the target card. However with Fallen Oni you also get a unit on the battlefield that makes for a cheap blocker.
This page written for Eternal version 1.22, Card Sets 1 & 2
Last updated: 19-07-2017