SlyFlourish, producer of several books and tips for DM, noted that at level 1 it was a deadly grind.
It might be intentional but I didn't like it that way so I've up-leveled or made them level 2 very quickly.
It was a grind at level 2 but a GOOD grind. At level 1 it's deadly.
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With that in mind, consider leveling them up right away, and that should help many of the issues. At the very least, giving each character a significant percentage of their hitpoints by leveling should help them survive more.
You can see more of SlyFlourish's suggestions for a smooth run of the Greenest in Flames section here
"Sentient magic items function as NPCs under the DM's control," says the DMG (214). They have their own personalities; they have Intelligence, Wisdom, and Charisma scores. If they weren't trapped inside swords and boots, they'd be out living their lives and making decisions just like everybody else.
The PHB (122) says celestials and fiends are locked into their alignment, but "for many thinking creatures, alignment is a moral choice." Even the races created by evil gods have free will, and can pursue good alignments if they choose.
I gather that HotDQ isn't very specific about Hazirawn's personality, but if there really is a sentient being in there, you should be able to change its alignment the same way you'd convince a wrathful orc or bloodthirsty gnoll to turn toward Good. It could be a philosophical argument, or a series of object lessons that show the sword the error of its ways. Your chances of success would depend on Hazirawn's own force of personality, which is why I brought up that Charisma score.
Although RAW implies that people can change their alignment, it's up to your DM whether they actually can, and whether an outside agent can impose that kind of change. In any case, my answer is that a sentient magic item, by virtue of possessing a personality, should be treated like a person.
(If you can't hack the philosophical argument angle, you might try to get Hazirawn to draw Balance from the Deck of Many Things...)
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Well, anything that can talk to someone can influence them. Just like any character, Hazirawn can influence its wielder by the simple expedient of giving them advice, or suggestions, or even just by chatting to them. This method relies solely on your creativity and powers of persuasion to roleplay Hazirawn well and manipulate your player into doing things.
While Hazirawn doesn't mention anything about taking over its wielder, it is a sentient item, and the DMG section on sentient items includes this paragraph on page 216:
Hazirawn has an alignment listed, but if you want to use these rules, you will still need to decide on its Charisma score. Note that some of the lesser demands and consequences listed here may be just as useful for influencing a player as taking control of them. If Hazirawn's wielder has come to rely on its awesome power, threatening them with the loss of some or all of that power is highly likely to be an effective way to influence them.