If creatures are taken unawares, they are Surprised. Surprised creatures cannot act until they become aware of their adversaries, nor can they React to anything during that round. Surprised creatures grant Combat Advantage to creatures they are unaware of.
Even after becoming aware of their adversaries, you can only take 1 Action during the remainder of that round, and that Action cannot be a Master Action.
Combat very rarely takes place on nice floor or featureless terrain. Many spells create difficult terrain, but so does environmental effects, such as attempting to find in sand, overgrowth, or knee-high water.
Every foot of movement through difficult terrain costs 1 extra foot. This is true even if multiple things in the space count as difficult terrain.
At any time during combat, you can move through the area that a friendly creature or ally is currently in, but cannot end there.
You cannot move through a hostile creature's space, unless that creature is 2 size categories larger than you (or larger!), and you still cannot end your turn there. Moving through a hostile creature's space always counts as difficult terrain.
Any Small sized creature or larger can end their turn in the space of a tiny sized creature or smaller with no problems.
Creatures of tiny size and smaller take up less room. If you are using a combat grid, see below to find out how much space a creature takes.
Note, this assumes you are using a 5-foot square for your combat grid, and that the creatures are attempting to fight. Obviously, you could fit many cats (tiny creatures) in a 5 foot square, but not more than 2 Imps attempting to attack a warrior.
| Size | Space on a Grid |
|---|---|
| Fine | 8 creatures per square |
| Diminutive | 4 creatures per square |
| Tiny | 2 creatures per square |
| Small | 1 square |
| Medium | 1 square |
| Large | 4 squares |
| Huge | 8 squares |
| Gargantuan | 16 squares |
| Colossal | 32 squares |
Occasionally, you have cause to throw what is called a Splashing Weapon. Vials of Holy Water thrown at the undead are the most common splashing weapon. All splash weapons are considered to be range 15/30, unless otherwise noted.
When you throw a splashing weapon, you can target a square. You then make a d20 test using Dexterity; unmodified by any bonus. The DC of this test is 15. If you hit, the Splashing weapons lands on-target, and you can select any corner of the square you were targeting to be the center of the 5-foot burst of the splashing weapon. Alternatively, you can select to have the splash weapon strike a foe in the square that you were aiming at; instead causing only the foe to take the effect, as well as a number of points of bludgeoning damage equal to your Strength Modifier (minimum of 0).