Yakshayuddha Parva is the thirty fifth upa parva included in the third maha parva, named as Vana parva or Aranyaka parva in the epic Mahabharat. The original epic was composed by Vyasa in Sanskrit. Vyshampayana had narrated to Janamejaya the historical events, happened in the forests on his queries.
How was the Gandhamadana forest?
धर्मारण्यं हि तत्पुण्यमाद्यम् च भरतर्षभ । यत्र प्रविश्ट मात्रो वै पापेभ्यो विप्रमुच्यते ॥
Meaning: O Bull Bharatha! It was also a forest full of Dharma, where people reaching there get rid of their sins. It was a holy place.
What had Arshtishena Rishi advised Yudhishtira?
Arshtishena Rishi had his Ashrama near Mount Shweta near Gandhamdana forest. When Yudhishtira reached Arshtishena Ashram, he welcomed them and told them many valuable secrets of human life.
पिता माता तथैवाग्निर्गुरुरात्मा च पंचमः । यस्मै ते पूजिताः पार्थ तस्य लोक वुभौ जितौ ॥
Meaning: O Partha! Those who worship father, mother, fire, teacher and inner-soul (say God or Paramatma), they could win the worlds, this world and heaven.
Yudhishtira desired to win in the forthcoming Mahabharat war and Arshtishena Rishi was indirectly hinting him about his victory.