"Each step through the muddy bog felt like twenty. By the time we reached the enemy, we were exhausted. But the elves, they were still fresh and moving about without any trouble. How?"Terrain Walk is a first-level Nature Magic spell in Heroes of Might and Magic IV that costs two spell points and gives the target the ability to walk on all terrains without penalty.
Difficult terrains don't only slow your troops down on the strategy map, but also on the battle map. This can make it difficult for non-flying melee units to close with their opponents. There is no skill that solves this problem (the Pathfinding skill only works on the strategic map), but this spell does. Use it to help your heroes and melee creatures close more quickly with those pesky ranged attackers over disagreeable terrain.
This spell is of extremely limited use. Firstly it only has an effect on walking creatures and the Preserve has a preponderance of flying creatures (sprites, griffins, phoenixes, mantises). This means that, excluding creature portal creatures, the spell is only going to be cast on wolves, white tigers and unicorns, and elves and griffins are often chosen over the latter two of these.
This limitation is compunded by the fact that Speed (which has the added advantage of increasing combat speed as well as movement) is almost as effective as Terrain Walk for these walkers. Even on the slowest combat terrain, swamp, the difference in increased combat movement between this spell and Speed is small enough that there is only a difference in going from one side of the battlefield to the other for Unicorns (the fastest creature) - it takes 3 turns from the front row to the back row with Speed and two turns with Terrain Walk. There is no turn difference for any of the other creatures or for the Unicorns going from the front to the front row. The difference is even smaller on the less movement restrictive terrain types.
Overall, use this spell only if you don't have anything better to cast.