Legolas is using Quick Strike to a Wargs with 2hp left. Flipping the Shadow card would reveal the Wolf Rider. So even though the Wargs are vanquished by the Quick Strike, does Wolf Rider remain in play to make an attack before going to the top of the Encounter Deck?

You would normally play and resolve the effect of Quick Strike before the targeted enemy has a chance to attack (i.e., usually, but not always, in one of the action windows during the Encounter phase as per Core rules p. 31, or before or after step 1 in the Combat phase as per pages 18 and 31 of the Core rules). If a hero kills the enemy with Quick Strike before it has a chance to attack, then the Shadow card would be discarded without being resolved because the enemy was unable to attack.

Also, I highly recommend reviewing the the most recent version of the official FAQ, which answers this and many other questions you may have about the game. From p. 13 of the FAQ:

Q: If an enemy does not attack or its attack is canceled, what happens to its shadow card(s)?

A: At the end of the combat phase, discard each unresolved shadow card in play. (Do not resolve the effects on these shadow cards)

I hope this helps. Welcome to the addiction and these boards… have fun!

Thank you very much! Will do!

No, the Shadow Card does not go into effect. When starting the Combat Phase, shadow cards are dealt to the enemies as the first thing in that phase. After that, players can immediately take actions. So you can play Quick Strike and attack an enemy without even revealing the shadow card, which is great because that eliminates a card from the encounter deck with no harm.