The current Legends client reportedly contains 409 slash commands, and a recent tool extracted them directly from the client tables. Rather than dump hundreds of obscure/debug-style entries that you will probably never use, here are the known player-useful commands, grouped by purpose.

Core character and movement commands
Command What it does
/help Shows available command help
/loc Displays your current Y, X, Z coordinates
/target <name> Targets a nearby NPC/player/corpse
/tar <name> Short form of /target
/assist Targets your current target's target
/assist <name> Assists a specific player
/assist off Prevents assist from automatically starting attack
/assist on Enables attack-on-assist
/follow Follows your current target
/follow <name> Follows a named player
/stand Stand up
/sit Sit down
/camp Camp to character select
/quit Exit/camp command behavior depending on context
/rewind Attempts to move you out of stuck geometry
/consider Considers your target
/con Common abbreviation for consider
/who Lists players in your current zone
/who all Searches across the server
/who <name> Searches for a particular player
/who all friends Shows online friends
/charinfo Character/bind-related information
/surname <name> Set/change surname when eligible

Legends specifically confirms /target works on mobs, players, and corpses within target range, and vertical distance does not count against that targeting range.

Combat
Command What it does
/attack on Start autoattack
/attack off Stop autoattack
/assist Assist current target
/doability <skill> Activate a combat/general ability
/doability Bash Example: Bash
/doability Slam Example: Slam
/doability Taunt Example: Taunt
/doability Kick Example: Kick
/disc <name> Activate a discipline/ability by name
/yell Calls for help / unlocks the encounter for outside assistance
/autofire Toggles automatic ranged attacks where applicable
/stopcast Stops your current spell cast
/stopsong Stops bard song
/stopdisc Cancels/stops an applicable discipline
/shield <player> Uses the shield ability where available

Legends supports ability names directly, such as /doability Bash; its current macro system also uses /disc <discipline name>.

For your Barbarian Warrior, these are particularly useful:

/attack on
/attack off
/assist
/doability Slam
/doability Taunt
/doability Cleave
Spell commands
Command What it does
/cast 1 Cast spell gem 1
/cast 2 Cast gem 2, etc.
/memspellset <name> Load a saved spell set
/memspellslot ... Memorize/manage an individual spell slot
/book Open spellbook where supported
/stopcast Interrupt your current cast
/alt activate <#> Activate an AA by ID
/tgb Toggle targeted group buff behavior

Legends macro documentation explicitly confirms /cast, /memspellset, /memspellslot, and /alt activate.

Macro/social commands

These are extremely useful in Legends:

Command Purpose
/hotbutton <name> <color> <text> Creates a one-line macro hotbutton
/pause <#> Delay in tenths of a second
/timer <#> Adds a visual cooldown overlay to a macro
/doability <name> Execute a skill
/cast <#> Cast a spell gem
/disc <name> Use discipline
/autoinventory Put cursor item into first available inventory slot

An important quirk: in a macro such as

/pause 20, /cast 1

the /cast 1 happens first, then the two-second pause occurs. Legends documentation explicitly confirms this behavior.

You can also use substitution variables:

Variable Expands to
%T Current target's name
%S Target subject pronoun
%O Target object pronoun
%P Target possessive pronoun
%R Target race
%M Your pet's name

For example:

/g Attacking %T!

Legends documents all six of those substitutions.

Communication
Command Function
/say <text> Local say
/s <text> Short form where recognized
/tell <name> <text> Private tell
/t <name> <text> Short form of tell
/ttell <text> Tell your current targeted player
/tt <text> Short form of targeted tell
/gsay <text> Group chat
/g <text> Short group-chat form
/guildsay <text> Guild chat
/gu <text> Guild chat shortcut
/ooc <text> Out-of-character chat
/shout <text> Zone-wide shout
/auction <text> Auction channel
/emote <text> Custom emote
/reply Reply to latest tell
/join <channel> Join chat channel
/leave <channel> Leave chat channel
/list List joined channels
/autojoin ... Configure channels to join automatically

EverQuest's chat system also allows numbered channels such as:

/1 Hello
/2 Hello

for whatever channel occupies that number. The standard EQ chat-channel syntax is documented historically and remains part of the Legends client command set.

Group and raid commands
Command Function
/invite <name> Invite player to your group
/disband Leave/disband from group
/follow Follow target
/assist Assist target
/marknpc Mark targets where supported
/xtarget Extended-target commands
/xtarget save Save Extended Target configuration
/yell Open encounter to outside help

Legends specifically supports /xtarget save; recent Legends macro tooling uses it to establish tank, assist, and marked-target assignments.

Inventory and corpse commands
Command Function
/autoinventory Move cursor item into inventory
/hidecorpse all Hide all corpses
/hidecorpse looted Hide corpses you've already looted
/hidecorpse none Restore corpse display
/loot Loot targeted corpse where applicable
/destroy Cursor-item destruction functionality depending on UI/client state

/autoinventory is especially useful in Legends macros and is explicitly documented by the Legends community.

Notes and quest tracking

You asked about this earlier, and one correction is worth emphasizing:

/notes

is the documented Legends command for keeping personal notes. The Legends wiki specifically recommends it for tracking classic quests.

Pet commands

These will matter a lot once you're using your Necromancer and later your Shaman pet.

Command Effect
/pet attack Attack your current target
/pet kill <name> Attack a named target
/pet back off Clear pet hate list and stop attacking
/pet follow me Follow you
/pet guard me Follow/guard master mode
/pet guard here Stay at current location
/pet sit down Sit
/pet taunt on Enable pet taunt
/pet taunt off Disable pet taunt
/pet taunt Toggle taunt
/pet health Print pet HP/buff information
/pet target Target your own pet
/pet leader Report the pet's owner
/pet get lost Dismiss/kill your pet
/pet as you were Reset some current pet orders
/pet hold Hold mode; requires Companion's Discipline
/pet ghold Greater Hold toggle
/pet ghold on Enable Greater Hold
/pet ghold off Disable Greater Hold

Those are directly documented for Legends.

For your Warrior/Shaman/Necro setup, you'll probably want a pet attack button such as:

/assist
/pet attack

and a panic button:

/pet back off
Bard commands

If you ever add Bard:

/melody ...
/queuemelody ...

Both are specifically mentioned in the Legends macro documentation.

Interface and convenience commands

Depending on the client/UI configuration, the Legends client command table also includes commands for things such as:

/map
/log
/loadskin
/loadspells
/loadsocials
/chatfontsize
/shownames
/showspelleffects
/filter
/key
/friend
/ignore
/anonymous
/roleplay
/inspect
/duel
/time
played