Of the glacially sculpted hills that surround Abergavenny, Skirrid (486m) is the most dramatic looking and has a history to match. A cleft in the rock near the top was once believed to have split open at the exact time of Christ's death and a chapel was built here on what was considered a particularly holy place (a couple of upright stones remain). During the Reformation as many as 100 people would attend illegal Catholic Masses at this remote spot.
You can trek here from Llanvihangel Crucorney (5.5 miles) or take the B4521 from Abergavenny to the lay-by at the base of the hill (4 miles). It's a steep climb from here through the woods on a track that can be muddy; wear sensible shoes. Once you clear the tree line the walk is less steep, with a final climb right at the end to the summit, where you'll be rewarded with extravagant views.