quote:Standing over 100 feet in height with a trunk over 20 feet in diameter at its base, the Tree of Wailing Souls has literally thousands of branches and roots. The Tree of Wailing Souls has 30 Hit Dice and a THAC0 of 1. The trunk of the tree has 150 hit points and an Armor Class of 3 (from the toughness of the bark). It takes 1d8 points of damage to sever a limb or root, each of which is considered to be AC 5, but such damage does not count against the trunk’s total. The Tree of Wailing Souls is largely resistant to magic (75% Magic Resistance) and wholly immune to magical and nonmagical fire.
I try to change in 3.5 : _ THACO of 1 -> BAB + 20 ? _ 3 AC -> 17 AC _ AC 5 for a limb or root -> 15 AC _ 75% Magic Resistance -> 10 + DV = 30 % ?
In 2E, THAC0 was "To Hit Armor Class 0". Armor Class ranged from 10 (no armor) to -10 (the best armor imaginable). To determine your to-hit number, you subracted the enemy's AC from your THAC0 (which varied per level), and then you tried to roll under that number. So if your THAC0 was 20, and your enemy was AC 5, you needed to roll 15 or less to hit.
You may want to look at the 2E to 3E conversion book. It can be downloaded from this page.