hi, my name is Itay and I created some cards and I want you to tell your opinion about them. Also, I don't really think that the names that I gave them are good, so I would love to hear your suggestions
My opinion on your cards is as follows. Remember the Dead is kind of awkward because I don't know why I'd play it: if I have a creature with good abilities, why would I sacrifice it to get an enchantment with those same abilities? The effect of Bad Decisions lasts until "the end of your next turn," which is an odd length of time, but more importantly it has hidden utility as a kill spell when you use it on an opponent's creature during your second main phase. Since this is in it's okay though. Sea Reviver is fine-- Magic has been moving away from landwalk recently because it's entirely dependent on what your opponent is running, but the kicker means islandwalk will always be relevant.
In general, all of these designs have memory issues: you have to remember which creature card you sacrificed with Remember the Dead, even when there are five creature cards in your graveyard, for instance. Or you have to remember which of your opponent's lands you turned into an Island. There are fixes for all of these-- for instance, Bad Decisions could be retooled as an Aura like Parasitic Implant
And additional note concerning Remember the Dead: I like the idea behind the card, but it's too complex for a common imho. Also to solve the issues voltaic-qui had with your card as well as to be more in flavor of the card's name, I think it shouldn't sacrifice a creature but rather exile a creature card from your graveyard.
Doomguard: The +2 ability is very strong for its mana cost, and is probably stronger than the -2 since that only hits Demons. It also has two issues in that the life payment doesn't work the way it's written, and that you can't use it if there are no creatures on the battlefield, or if the only creatures on the battlefield are your own. You could fix both these problems with wording like "You may pay 2 life. If you do, destroy target creature." As I said earlier the -2 is fairly weak in comparison, and clashes heavily with the also weak ultimate ability - using one ability just means less Demons for the other. One way you could buff the ult is by making it return ALL creature cards from your graveyard and making them into Demons - see the wording on Liliana, Death's Majesty's -3. It might be worth coming up with an ability that puts Demons / other creatures into your graveyard to get more use out of the ultimate.
Forest Avenger: You're rarely going to want to play a 4 mana 1/1, no matter what keywords it has. Deathtouch + first strike just say "do not block / attack into this creature", meaning your opponents will likely just ignore it, and first strike doesn't really appear in black enough to justify its inclusion on a black / green card, so you could remove first strike and increase the size of the creature, which would make it a lot more desirable. You could also shift the rarity down to uncommon, since a creature with no abilities other than keywords isn't really rare unless it's very strong.
Also: your names and typelines seem to be in a strange font, which might mean you're missing a font file. I don't really know anything about that but it might be worth checking to make sure you've got all the right files for your templates, or asking for help in the templates section of the forums.