The Entropy game mode originally comes from Descent 3. It is a team based game mode and works as follows:

• A level contains special rooms; these can be energy centers, repair bays and virus labs.
• Rooms are owned by teams.
• Rooms can be conquered by the opposing team.
• The game ends as soon as a team has no more rooms and doesn't have the resources to conquer one.
• To conquer an enemy room, a player must pick up a number of viruses and stay in that room for a certain amount of time without moving his ship (ship movement due enemy fire is permissible).
• If a player is in an enemy room, his shields get constantly damaged until he either has conquered the room, or leaves the room, or is destroyed.
• Viruses are created in virus labs (they work much like robot generators).
• To be able to pick up a virus, a player must increase his transport capacity by killing opposing players. For each kill, his transport capacity rises by two.

That's the basic game play. Now here's some details:

• Besides the usual fuel centers, Entropy has two new room types:
• Virus labs produce viruses in a constant production time (can be set in the Entropy options menu). In D2X-XL, viruses look just like the green hoard orbs. Virus labs are marked by a sign similar to the flash and wrench signs, but contain a green orb .
• Repair bays replenish a ship's shields much like fuel centers replenish lost energy. They are marked with a wrench symbol .
• Room ownership is signaled by the room's walls being covered with a team specific texture (D2X-XL currently uses the flag goal textures).
• If a player conquers a room, his virus count is decreased by the number of viruses that were required to conquer that room.
• Optionally, if a player conquers a room, his virus transport capacity is decreased by a certain amount (can be set in the Entropy options menu).
• Viruses are owned by teams, and a team can only pick up their own viruses (from virus labs or dropped by team players).
• If a player gets shot down, he loses all viruses he has on board. Depending on game settings, these viruses stay floating around for a while or forever or vanish.
• If a player gets shot down, his transport capacity is reset to zero.
• If a team loses its last virus lab, another of its rooms is converted to a virus lab; provided the team still has at least one other room.
• If a team conquers a virus lab, depending on game settings, previously converted virus labs are reverted to their original type.

In entropy games, the HUD shows special information in the upper right corner of the screen:

• In the second row, a virus symbol is displayed
• Right to the virus symbol, the number of viruses carried and the virus transport capacity in square brackets are shown .
• If a player can conquer a room, that line turns yellow .
• If a player is conquering a room, the remaining time until take over is displayed in red digits right to the virus display .
• Right of the team scores, the number of rooms each teams owns and the total number of rooms in the level are displayed .
• Keys (red, blue, gold) and bombs are not displayed.

Here's a list of all Entropy options:

Viruses required for capture:		# of viruses on board required to conquer a room. (Default: 5)
Time required for capture:		Time in seconds required to conquer a room. (Default: 3 s)
Max. virus capacity:		Maximum number of viruses a player can carry (0: unlimited). (Default: 0)
Increase virus cap. on kill by:		Amount by which a players virus transport capacity will be increased when killing an opponent. (Default: 2)
Decrease virus cap. on capture by:		Amount by which a players virus transport capacity will be decreased when conquering a room. (Default: 5)
Virus production time:		Time in seconds a virus labs needs to produce a virus. (Default: 5 s.
Virus life span:		Life span in seconds of a virus that is jettisoned or dropped by a dieing player (0: unlimited). (Default: 0)
Fuel fill rate:		Amount of enegery a player gains per second when staying in a fuel center. (Default: 25)
Shield fill rate:		Amount of shields a player gains per second when staying in a repair bay. (Default: 11)
Shield damage rate:		Amount of shields a player loses per second when staying in a hostile room. (Default: 11)
Play conquer warning sound:		If checked, every player in the mine hears a siren when someone is trying to capture a room. (Default: No)
Revert rooms converted to labs:		If checked, all converted virus labs of a team are reverted back to their original state when that team conquers another virus lab. (Default: No)
Inhibit suicide:		If checked, players cannot commit suicide (neither accidentally nor intentionally). (Default: No)
Apply player handicap:		If checked, damage inflicted by a player with more kills than his target is decreased (up to 50%). (Default: No)
Room texture handling
keep room textures   If a room is conquered, only the sides with a team texture are covered with the new owner's team texture.   Override room textures   All visible sides of a team's room are covered with the team's texture.   Colorize rooms   The inside of team rooms is lit with the team's color. (Default)
Brighten rooms:		If checked, all rooms owned by a team are brightly lit. You must chose "Override room textures" to let this option have an effect. (Default: No)
Virus stability: destroy
when dropped:   Viruses jettisoned or dropped by a dieing player will vanish immediately. (Default)   when touched by enemy player:   Viruses touched by a player of the opposing time will vanish.   when touched while not in lab:   Virus that do not reside in a virus lab of their own team will vanish when being touched by an enemy player. (Actually, this is meant to keep viruses that have been produced by a virus lab and not been picked up yet from vanishing, but if a virus is dropped and floats into a virus lab of its own team, it will be protected as well. This is a feature, not a bug. As a virus lab will only produce a virus if none is in it, this is not a real problem though).   never:   Viruses will stay around until picked up by a player from its team or their life span expires.

DLE-XP has been enhanced to support Entropy levels. To build an entropy level, create a new level and chose "D2X-XL" as level type (this level type is incompatible to other Descent versions and inhibits such levels from being loaded by them at all).

Now you can:

• Insert virus labs: These are simply robot generators. When starting an Entropy game, D2X-XL will turn them into virus labs.
• Insert repair bays.
• Insert team specific spawn locations by placing the player starts in segments of type "blue team" or "red team".
• Set a segment's owner:
  • neutral: Segment cannot be conquered
  • unowned: Segment doesn't belong to a team at start of match, but can be conquered
  • blue team: Segment belongs to the blue team at start of match
  • red team: Segment belongs to the red team at start of match
• Set a segment's group. As the biggest spatial entity D2 knows is a segment, it has no inherent concept of a room as humans would understand it. With the group property you can form rooms consisting of several segments. A room consists of all segments with the same group id. This implies than a room can consist of several locations in the level that are not directly connected. There can be up to 127 rooms in a level.

• D2 supports up to 70 special segments (fuel centers, repair bays, robot makers/virus labs), 20 of which can be robot makers/virus labs.
• Actually, though the group feature allows to combine arbitrary segments in a room, it is recommended to only have a single special segment (fuel center, repair bay, virus lab) per room.
• If making rooms consisting of several segments, some consistency should be kept in room size and spatial distribution i.e. a room should preferrably consist only of connected segments and not have several sections spread over the level. Of course, if thoughtfully used, having a room distributed over several areas in the level can make up for some interesting tactics.
• Note that D2X-XL's entropy mode supports rooms that are neutral when the match is started, but can be conquered. Using this feature thoughtfully can also provide some interesting tactical tweaks in game play.
• D2-Entropy uses the proximity bomb and smart mine weapon slots to store the players' virus count and virus capacity, so an Entropy level should not contain any of these weapons. If it does, they will be turned to shields.

The virus symbol is not part of the standard D2 texture set. To support level builders in building Entropy levels, I have created a pog file (custom texture file) containing the virus symbol. You can download it here. To use it in a level of yours, simply rename it to <mylevel>.pog (where <mylevel> is the filename part of the level filename <mylevel>.rl2 you created) and import it in your level's hog file with DLE-XP.