“Night Assailant” doesn’t have to be sent to the Graveyard by an effect, or by an effect controlled by your opponent; it just has to be sent to the Graveyard. “Night Assailant”’s effect activates when sent to the Graveyard as a cost for “Magic Jammer” or “Tribe-Infecting Virus”; “Night Assailant”’s effect activates after the current chain resolves.
“Night Assailant”’s effect activates when sent to the Graveyard by “Morphing Jar” or “Card Destruction”.
“Night Assailant”’s effect activates when sent to the Graveyard for a Ritual Summon.
When “Night Assailant” is sent to the Graveyard by the effect of “Last Turn”, FIRST resolve “Last Turn”’s effect of sending all cards on the field and in the hands to the Graveyard, SECOND activate and resolve the effect of “Night Assailant”, and THIRD Special Summon a monster by the effect of “Last Turn”.
You cannot select the “Night Assailant” that was itself sent to the Graveyard, so there is no infinite loop.
When multiple “Night Assailants” are sent to the Graveyard at the same time because of “Graceful Charity”, etc., both can be returned to your hand because “Night Assailant” cannot select itself but CAN select a different “Night Assailant”, even one that was sent to the Graveyard at the same time.
Both of “Night Assailant”’s effects target.
You cannot use “Night Assailant”’s second effect while “Necrovalley” is active.
If your opponent has your “Night Assailant” in his hand (because of “Exchange”) and it’s sent to the Graveyard, you (the owner) get the effect.
If you have “Forced Requisition” active (face-up) and 7 cards in your hand during the End Phase (and one of them is a “Night Assailant”), and there is another “Night Assailant” in your Graveyard, and you discard your “Night Assailant” because you have 7 cards, your opponent must discard because of “Forced Requisition”. Then you can activate the effect of “Night Assailant” to retrieve the other “Night Assailant”. Now you have 7 cards in your hand again so you can discard “Night Assailant”, and your opponent must discard again for “Forced Requisition”. You can continue doing this until your opponent loses all cards in their hand.