Surprise is determined, when applicable and including modifiers (the DM keeps track of these), with each party rolling a d6. 1-2 means the party is surprised, while anything else indicates no surprise. If one party is surprised, and the other is not, the party that has surprised the other has as many free segments to act as the difference between the die rolls (ie, the surprised party rolled a 1, their opponents rolled a 6 and hnow have 5 free segments to act before initiative is rolled.)
Segments are just another breakdown of increments of time. One year = 365 days. One hour = 60 minutes. One round = 10 segments. That’s it.
As to how it is used in regards to spells, the casting time is generally in segments. So, a typical third level spell has a casting time of 3, which means three segments. Now, combine this with the initiative roll and consider the initiative roll to be the segment count. So, if the adventureres rolled a 5 and the monsters rolled a 9 this would mean that on the characters would start their actions on the 5th segment of the round and the monsters on the 9th. A mage casting fireball would start his casting on the 5th segment and would end on the 8th segment. Now, if the monsters had rolled a 7 instead of a 9 on initiative this means that they could would attack before the mage finishes casting his spell, thus allowing for the casting to be disrupted.
Weapon speeds work in a similar manner. If one the adventurers had a weapon with a speed of three his attack would occur on the 8th segment. With weapon speed it is possible to lose initiative yet still go first, or to win initiative and go last.
The only change here from the PHB, is we will roll d10 for both sides in initiative, not d6.
(thank you to Mock26 of the Wizards of the Coast community for this breakdown.)
Actions You Can Take During The Combat Round
A) Avoid Engagement (Flee/Slam Door/Use Magic to Escape, Etc…if possible)
B) Attempt to Parley
C) Await Action by Other Party
D) Discharge Missiles or Magical Device Attacks
E) Turn Undead
F) Cast a Spell
G) Close to Striking Range or Charge (see below Charging Rules)
H) Set Weapons Against Opponent Charge (ie, ready spear or polearm)
I) Strike a Blow!
J) Grapple or Hold
Initiative is NOT determined when charging. If the opponent is within 10’, you can charge, this gives you a +2 to hit, but you will no longer receive a dexterity for your AC for that round, characters without a dexterity bonus add one to their AC for the round, and any creature/PC with an AC 10 has no penalty on their AC.
The combatant in the charge attack with the longer weapon strikes first (no initiative). The combatant being charged is able to strike first if his weapon is longer. Combatants bracing weapons to receive a charge automatically strike first and do double damage if they hit.
Movement rates in combat do not change in regards to closing for combat (ie, 90’ feet/round/minute or 9’/segment/6 seconds)
When fleeing, the party’s base rate of movement is multiplied by 10. (ie 90’ feet is now traversed in one segment/6 seconds, and 900’ in round/one minute). As a general rule, this rate of movement can be maintained for as many rounds as the consitution of the player fleeing, or the rank of the PC with the lowest constitution if the party is fleeing en masse.
If the pursuing party is able to close within 10’, an encounter is (re)established for the slowest member of the party.
Death, Dying, Resurrection
Characters at 0 hit points are unconscious. They lose a hit point every round until another PC (or ally, henchment, hireling) can stabilize them. At -10, the PC is pushing up the medieval daisies, and their only hope lies with an extremely high level priest/cleric. They must be healed, magically or otherwise, to a positive number, before they are conscious.