Can I ambush a unit alone if arachnlod horror is in my base? Or does ambush only give reaction while I have a unit there?
No, you cannot use Ambush in that situation.
According to Rule 822.1.b and RiftJudge FAQ #9444, the Ambush keyword provides two things:
- Permission to play the unit to a battlefield where you already control units.
- The [Reaction] timing required to play that unit while the game state is Closed (such as during a showdown).
Because Arachnoid Horror is in your base and not at the battlefield, you do not control any units at that location. Therefore, you do not meet the requirement to use the Ambush keyword, and you cannot play the unit there using that ability.
Ambush does not grant you the ability to play a unit to an empty battlefield; it specifically requires you to already have a presence (a unit) at the target location to "ambush" from.