If a druid who had this staff were to wild shape into a bear, would the bears melee attack be able to trigger the necrotic effect and use a charge of the staff?

Quote from Razamis >>

If a druid who had this staff were to wild shape into a bear, would the bears melee attack be able to trigger the necrotic effect and use a charge of the staff?

 No, you have to attack with the staff to trigger the necrotic damage.

said druid would have to drop the staff first because when you beast shape everything worn and carried is basically invisible/protected/unusable while transformed. Read wild shape carefully. 

Does "Steel Wind Strike" enable the "On a Hit" effect with this staff?

Steel Wind Strike mentioned a "Melee Spell Attack" but also uses the "On a Hit" term in the spell...

Imagine if it does possible 5x 6d10 + an additional 3x 2d10 total damage output of 36d10 damage :D unless I'm reading the spell wrong or the rules doesn't enable the staff to activate.

the spell doesnt explicitly state you hit them with the weapon (which would allow the staff of withering to discharge) but any sane player or DM should assume that yes, you do hit them with the weapon, and such should be able to discharge the staff, although i sincerely feel this is a terrible idea

Steel Wind Strike is only for wizzard and ranger 

the staff is not 

This item pairs particularly well with Shillelagh! Take this as a Druid, or as a Nature Domain Cleric or a Pact of the Tome Warlock, and you're laughin'.

Does the disadvantage apply on every hit, or just when you expend a charge?

In my game, 100% yes

said druid would have to drop the staff first because when you beast shape everything worn and carried is basically invisible/protected/unusable while transformed. Read wild shape carefully. 

You might already have done so by now, but if not, you might wanna follow your own advice, because this is what it says:
"You choose whether your equipment falls to the ground in your space, merges into your new form, or is worn by it."

It then explains that any equipment can only be used or worn if your new form's anatomy allows it.

This would be a great weapon for a monk. If only they could take it...

Quote from MadFerretx >>

This would be a great weapon for a monk. If only they could take it...

Take a single level in Cleric or Druid; since they're wisdom based they pair well with Monk, and both have some solid benefits even if you only take 1-3 levels.

How many levels you should take will depend on how high a level you expect your campaign to reach, but a single level in Cleric gets you spellcasting and a Domain (which in turn grants extra spells known, and generally quite good 1st-level abilities). Druid only gives you spellcasting at 1st-level, but at 2nd-level you get Wildshape (a highly versatile ability) plus a Circle with 2nd-level ability (better/quicker wildshaping as Circle of the Moon, more spellcasting as Circle of the Land, other ways to spend wildshape as Spores or Stars etc.).

IMO you'll usually want to aim for a third level in either since in both cases it gets you six spell-slots to work with (four 1st-level, two 2nd-level) along with those 2nd-level spells and a good number of spell choices. But it really depends how high level you expect to get as a Monk; past 7th-level the features of Monk can be a bit "meh", so you're only really losing Ki points, but Monk 7/Cleric 3 or Monk 7/Druid 3 can be a really solid 10th-level character.

Plus since we're talking about the staff of withering here, a single level in either lets you attune to the staff, which you can then use as a Monk weapon. While it only has three charges, it triggers on a hit so you can pretty safely deliver a full extra 6d10 necrotic damage to a high value target (or 2d10 each to three targets etc.), forcing three saving throws to avoid Strength and Constitution disadvantage, and you can also throw some Stunning Strikes on top of that for good measure, since those also trigger on hit.

You can also technically make Warlock work but it requires you to have enough Charisma to multiclass (at least CHA 13) which is tricky using standard array or point buy. Even if you only have the bare minimum, you can still get quite a bit out of Warlock as well depending upon your choice of invocations and spells. Basically you want to pick things that don't require your Charisma for attack rolls/saving throws, Pact of the Chain plus some of the corresponding invocations (Gift of the Ever-Living Ones and/or Investment of the Chain Master) would be my recommendation, Devil's Sight is also good (pairs nicely with Way of Shadow). It's a much trickier build though, as you're mostly taking Warlock for utility rather than power if you're Charisma is bad.

The legacy version of this staff from 2014 was restricted to Cleric, Druid and Warlock.  The 2024 version of the staff (from the DMG) doesn't appear to have that restriction though - it's just an attunement weapon that anyone can use.  Maybe that change was missed when the magic items database was updated for 2024?