My point is, you may have other cards you could put in instead.
Posting your collection will help if you need an idea what to do with the cards in your collection. Here’s how to do it:
Check my spreadsheet for a rough rating on these cards:
https://docs.google.com/spreadsheets/d/1gNLJfRRg5Eiid7mHZ_s0O4lVuVeTuVGVaFe02ejxQwA
These ratings are bound to shift from time to time.
Here’s a more detailed overview of each legendary you mentioned:
The Beast: pretty bad, dust later.
Ok to play casually if you’re missing cards to put in. Later on you’ll use other cards.
Ysera: Good, Keep.
Decent late game control legendary. Keep, since it’s also from the Classic set.
Making Mummies: OK.
You’ll need Reborn cards from the same set (Saviors of Uldum) to really use this one (most are common/rare). Overall it offers interesting deckbuilding choices, can outright win games at lower ranks.
Grand Empress Shek’zara: Niche
Has synergy with other cards that put stuff in your deck eg: Ticket Master. Could be good, I have not seen too much of her.
Instructor Fireheart: Ok
Can provide decent value discovering multiple spells in one turn and being able to play them. Pretty playable for most Shaman decks.
Lady Vashj: Niche
The spell damage on a 4/3 body isn’t anything spectacular, and the Prime form can be pretty weak the turn you play it. You can put her in, but don’t expect amazing things.
Vessina: Good
Form a wide board, play an Overload card, and buff up your minions attack, for 2 turns! Very good for an aggressive gameplan.
Captain Greenskin: Deck Staple, Keep
Any deck running weapons and pirates, Greenskin is always a consideration. Keep. Anytime you see Doctor Krastinov in a decklist, you can put in Greenskin, the cards are very similar.
Malygos: Deck Staple, Keep
A card that has always found a way into some sort of combo deck since forever. Classic Set staple. You never disenchant this unless you really want to limit your collection.
Don’t overvalue the importance of legendaries. It’s more worth to craft some really good commons and rares to be able to try out more deck archetypes. Let’s say, use 500 dust. That can get you 3 rares and 5 commons for example, and if they are really good ones, that ups the consistency of your whole deck by quite a bit.
Whereas you can spend 1600 dust on a legendary and you don’t draw it at all in your games.